diff --git a/ownlang-core/src/main/java/com/annimon/ownlang/exceptions/UnknownClassException.java b/ownlang-core/src/main/java/com/annimon/ownlang/exceptions/UnknownClassException.java
index 8c2bed3..d963c25 100644
--- a/ownlang-core/src/main/java/com/annimon/ownlang/exceptions/UnknownClassException.java
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/exceptions/UnknownClassException.java
@@ -6,11 +6,6 @@ public final class UnknownClassException extends OwnLangRuntimeException {
     
     private final String className;
 
-    public UnknownClassException(String name) {
-        super("Unknown class " + name);
-        this.className = name;
-    }
-
     public UnknownClassException(String name, Range range) {
         super("Unknown class " + name, range);
         this.className = name;
diff --git a/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassDeclaration.java b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassDeclaration.java
new file mode 100644
index 0000000..6f0e50e
--- /dev/null
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassDeclaration.java
@@ -0,0 +1,24 @@
+package com.annimon.ownlang.lib;
+
+import java.util.List;
+
+public record ClassDeclaration(
+        String name,
+        List<ClassField> classFields,
+        List<ClassMethod> classMethods) implements Instantiable {
+
+    /**
+     * Create an instance and put evaluated fields with method declarations
+     * @return new {@link ClassInstance}
+     */
+    public ClassInstance newInstance(Value[] args) {
+        final var instance = new ClassInstance(name);
+        for (ClassField f : classFields) {
+            instance.addField(f);
+        }
+        for (ClassMethod m : classMethods) {
+            instance.addMethod(m);
+        }
+        return instance.callConstructor(args);
+    }
+}
diff --git a/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassField.java b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassField.java
new file mode 100644
index 0000000..469c6b0
--- /dev/null
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassField.java
@@ -0,0 +1,7 @@
+package com.annimon.ownlang.lib;
+
+public record ClassField(
+        String name,
+        EvaluableValue evaluableValue
+) {
+}
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassInstanceValue.java b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassInstance.java
similarity index 65%
rename from ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassInstanceValue.java
rename to ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassInstance.java
index 3def100..75c1292 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassInstanceValue.java
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassInstance.java
@@ -1,16 +1,18 @@
 package com.annimon.ownlang.lib;
 
+import com.annimon.ownlang.exceptions.OwnLangRuntimeException;
 import com.annimon.ownlang.exceptions.TypeException;
 import java.util.Objects;
 
-public class ClassInstanceValue implements Value {
-    
+public class ClassInstance implements Value {
+
     private final String className;
     private final MapValue thisMap;
     private ClassMethod constructor;
-    private UserDefinedFunction toString;
+    private ClassMethod toString;
+    private boolean isInstantiated;
 
-    public ClassInstanceValue(String name) {
+    public ClassInstance(String name) {
         this.className = name;
         thisMap = new MapValue(10);
     }
@@ -19,31 +21,33 @@ public class ClassInstanceValue implements Value {
         return thisMap;
     }
 
-    public String getClassName() {
-        return className;
+    public void addField(ClassField f) {
+        thisMap.set(f.name(), f.evaluableValue().eval());
     }
 
-    public void addField(String name, Value value) {
-        thisMap.set(name, value);
-    }
-
-    public void addMethod(String name, ClassMethod method) {
-        if (name.equals("toString")) {
-            toString = method;
-        }
+    public void addMethod(ClassMethod method) {
+        method.setClassInstance(this);
+        final String name = method.getName();
         thisMap.set(name, method);
         if (name.equals(className)) {
             constructor = method;
+        } else if (name.equals("toString")) {
+            toString = method;
         }
     }
 
-
-    public void callConstructor(Value[] args) {
+    public ClassInstance callConstructor(Value[] args) {
+        if (isInstantiated) {
+            throw new OwnLangRuntimeException(
+                    "Class %s was already instantiated".formatted(className));
+        }
         if (constructor != null) {
             CallStack.enter("class " + className, constructor, null);
             constructor.execute(args);
             CallStack.exit();
         }
+        isInstantiated = true;
+        return this;
     }
 
     public Value access(Value value) {
@@ -53,15 +57,16 @@ public class ClassInstanceValue implements Value {
     public void set(Value key, Value value) {
         final Value v = thisMap.get(key);
         if (v == null) {
-            throw new RuntimeException("Unable to add new field "
-                    + key.asString() + " to class " + className);
+            throw new OwnLangRuntimeException(
+                    "Unable to add new field %s to class %s"
+                            .formatted(key.asString(), className));
         }
         thisMap.set(key, value);
     }
 
     @Override
     public Object raw() {
-        return null;
+        return thisMap;
     }
 
     @Override
@@ -77,9 +82,9 @@ public class ClassInstanceValue implements Value {
     @Override
     public String asString() {
         if (toString != null) {
-            return toString.execute(new Value[0]).asString();
+            return toString.execute().asString();
         }
-        return className + "@" + thisMap;
+        return className + "@" + thisMap.asString();
     }
 
     @Override
@@ -100,7 +105,7 @@ public class ClassInstanceValue implements Value {
         if (obj == null) return false;
         if (getClass() != obj.getClass())
             return false;
-        final ClassInstanceValue other = (ClassInstanceValue) obj;
+        final ClassInstance other = (ClassInstance) obj;
         return Objects.equals(this.className, other.className)
                 && Objects.equals(this.thisMap, other.thisMap);
     }
diff --git a/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassMethod.java b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassMethod.java
new file mode 100644
index 0000000..643b700
--- /dev/null
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ClassMethod.java
@@ -0,0 +1,60 @@
+package com.annimon.ownlang.lib;
+
+import java.util.Objects;
+
+public class ClassMethod implements Function {
+    private final String name;
+    private final Function function;
+    private ClassInstance classInstance;
+
+    public ClassMethod(String name, Function function) {
+        this.name = name;
+        this.function = function;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setClassInstance(ClassInstance classInstance) {
+        this.classInstance = classInstance;
+    }
+
+    @Override
+    public Value execute(Value... args) {
+        ScopeHandler.push();
+        ScopeHandler.defineVariableInCurrentScope("this", classInstance.getThisMap());
+
+        try {
+            return function.execute(args);
+        } finally {
+            ScopeHandler.pop();
+        }
+    }
+
+    @Override
+    public int getArgsCount() {
+        return function.getArgsCount();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj == this) return true;
+        if (obj == null || obj.getClass() != this.getClass()) return false;
+        var that = (ClassMethod) obj;
+        return Objects.equals(this.name, that.name) &&
+                Objects.equals(this.function, that.function);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(name, function);
+    }
+
+    @Override
+    public String toString() {
+        return "ClassMethod[" +
+                "name=" + name + ", " +
+                "function=" + function + ']';
+    }
+}
diff --git a/ownlang-core/src/main/java/com/annimon/ownlang/lib/EvaluableValue.java b/ownlang-core/src/main/java/com/annimon/ownlang/lib/EvaluableValue.java
new file mode 100644
index 0000000..bc90ba0
--- /dev/null
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/lib/EvaluableValue.java
@@ -0,0 +1,6 @@
+package com.annimon.ownlang.lib;
+
+public interface EvaluableValue {
+
+    Value eval();
+}
diff --git a/ownlang-core/src/main/java/com/annimon/ownlang/lib/RootScope.java b/ownlang-core/src/main/java/com/annimon/ownlang/lib/RootScope.java
index 0cdcd81..19682bf 100644
--- a/ownlang-core/src/main/java/com/annimon/ownlang/lib/RootScope.java
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/lib/RootScope.java
@@ -8,11 +8,13 @@ import java.util.concurrent.CopyOnWriteArraySet;
 final class RootScope extends Scope {
     private final Map<String, Value> constants;
     private final Map<String, Function> functions;
+    private final Map<String, ClassDeclaration> classDeclarations;
     private final Set<String> loadedModules;
 
     RootScope() {
         functions = new ConcurrentHashMap<>();
         constants = new ConcurrentHashMap<>();
+        classDeclarations = new ConcurrentHashMap<>();
         constants.put("true", NumberValue.ONE);
         constants.put("false", NumberValue.ZERO);
         loadedModules = new CopyOnWriteArraySet<>();
@@ -71,6 +73,18 @@ final class RootScope extends Scope {
     }
 
 
+    public ClassDeclaration getClassDeclaration(String name) {
+        return classDeclarations.get(name);
+    }
+
+    public void setClassDeclaration(ClassDeclaration classDeclaration) {
+        classDeclarations.put(classDeclaration.name(), classDeclaration);
+    }
+
+    public Map<String, ClassDeclaration> getClassDeclarations() {
+        return classDeclarations;
+    }
+
     public Set<String> getLoadedModules() {
         return loadedModules;
     }
diff --git a/ownlang-core/src/main/java/com/annimon/ownlang/lib/ScopeHandler.java b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ScopeHandler.java
index a61d6ca..f82af08 100644
--- a/ownlang-core/src/main/java/com/annimon/ownlang/lib/ScopeHandler.java
+++ b/ownlang-core/src/main/java/com/annimon/ownlang/lib/ScopeHandler.java
@@ -28,6 +28,11 @@ public final class ScopeHandler {
         return rootScope.getFunctions();
     }
 
+    public static Map<String, ClassDeclaration> classDeclarations() {
+        return rootScope.getClassDeclarations();
+    }
+
+
     static RootScope rootScope() {
         return rootScope;
     }
@@ -75,6 +80,15 @@ public final class ScopeHandler {
     }
 
 
+    public static ClassDeclaration getClassDeclaration(String name) {
+        return rootScope.getClassDeclaration(name);
+    }
+
+    public static void setClassDeclaration(ClassDeclaration classDeclaration) {
+        rootScope.setClassDeclaration(classDeclaration);
+    }
+
+
     public static boolean isVariableOrConstantExists(String name) {
         if (rootScope().containsConstant(name)) {
             return true;
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassDeclarations.java b/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassDeclarations.java
deleted file mode 100644
index b079ef3..0000000
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassDeclarations.java
+++ /dev/null
@@ -1,31 +0,0 @@
-package com.annimon.ownlang.lib;
-
-import com.annimon.ownlang.parser.ast.ClassDeclarationStatement;
-import java.util.Map;
-import java.util.concurrent.ConcurrentHashMap;
-
-public final class ClassDeclarations {
-
-    private static final Map<String, ClassDeclarationStatement> declarations;
-    static {
-        declarations = new ConcurrentHashMap<>();
-    }
-
-    private ClassDeclarations() { }
-
-    public static void clear() {
-        declarations.clear();
-    }
-
-    public static Map<String, ClassDeclarationStatement> getAll() {
-        return declarations;
-    }
-    
-    public static ClassDeclarationStatement get(String key) {
-        return declarations.get(key);
-    }
-    
-    public static void set(String key, ClassDeclarationStatement classDef) {
-        declarations.put(key, classDef);
-    }
-}
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassMethod.java b/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassMethod.java
deleted file mode 100644
index 5ad723e..0000000
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/ClassMethod.java
+++ /dev/null
@@ -1,27 +0,0 @@
-package com.annimon.ownlang.lib;
-
-import com.annimon.ownlang.parser.ast.Arguments;
-import com.annimon.ownlang.parser.ast.Statement;
-import com.annimon.ownlang.util.Range;
-
-public class ClassMethod extends UserDefinedFunction {
-    
-    public final ClassInstanceValue classInstance;
-    
-    public ClassMethod(Arguments arguments, Statement body, ClassInstanceValue classInstance, Range range) {
-        super(arguments, body, range);
-        this.classInstance = classInstance;
-    }
-    
-    @Override
-    public Value execute(Value[] values) {
-        ScopeHandler.push();
-        ScopeHandler.defineVariableInCurrentScope("this", classInstance.getThisMap());
-        
-        try {
-            return super.execute(values);
-        } finally {
-            ScopeHandler.pop();
-        }
-    }
-}
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/Classes.java b/ownlang-parser/src/main/java/com/annimon/ownlang/lib/Classes.java
deleted file mode 100644
index 488d546..0000000
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/lib/Classes.java
+++ /dev/null
@@ -1,36 +0,0 @@
-package com.annimon.ownlang.lib;
-
-import com.annimon.ownlang.exceptions.UnknownClassException;
-import java.util.HashMap;
-import java.util.Map;
-
-public final class Classes {
-
-    private static final Map<String, ClassInstanceValue> classes;
-    static {
-        classes = new HashMap<>();
-    }
-
-    private Classes() { }
-
-    public static void clear() {
-        classes.clear();
-    }
-
-    public static Map<String, ClassInstanceValue> getFunctions() {
-        return classes;
-    }
-    
-    public static boolean isExists(String key) {
-        return classes.containsKey(key);
-    }
-    
-    public static ClassInstanceValue get(String key) {
-        if (!isExists(key)) throw new UnknownClassException(key);
-        return classes.get(key);
-    }
-    
-    public static void set(String key, ClassInstanceValue classDef) {
-        classes.put(key, classDef);
-    }
-}
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/Parser.java b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/Parser.java
index 278dee5..1c8417f 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/Parser.java
+++ b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/Parser.java
@@ -815,7 +815,8 @@ public final class Parser {
             final var startTokenIndex = index - 1;
             final Arguments arguments = arguments();
             final Statement statement = statementBody();
-            return new ValueExpression(new UserDefinedFunction(arguments, statement, getRange(startTokenIndex, index - 1)));
+            final Range range = getRange(startTokenIndex, index - 1);
+            return new ValueExpression(new UserDefinedFunction(arguments, statement, range));
         }
         return variable();
     }
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/AssignmentExpression.java b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/AssignmentExpression.java
index a698284..e76c0af 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/AssignmentExpression.java
+++ b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/AssignmentExpression.java
@@ -1,12 +1,13 @@
 package com.annimon.ownlang.parser.ast;
 
+import com.annimon.ownlang.lib.EvaluableValue;
 import com.annimon.ownlang.lib.Value;
 
 /**
  *
  * @author aNNiMON
  */
-public final class AssignmentExpression extends InterruptableNode implements Statement {
+public final class AssignmentExpression extends InterruptableNode implements Statement, EvaluableValue {
 
     public final Accessible target;
     public final BinaryExpression.Operator operation;
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ClassDeclarationStatement.java b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ClassDeclarationStatement.java
index b8f3e94..482cfba 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ClassDeclarationStatement.java
+++ b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ClassDeclarationStatement.java
@@ -1,8 +1,6 @@
 package com.annimon.ownlang.parser.ast;
 
-import com.annimon.ownlang.lib.ClassDeclarations;
-import com.annimon.ownlang.lib.NumberValue;
-import com.annimon.ownlang.lib.Value;
+import com.annimon.ownlang.lib.*;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -28,9 +26,27 @@ public final class ClassDeclarationStatement implements Statement {
 
     @Override
     public Value eval() {
-        ClassDeclarations.set(name, this);
+        final var classFields = fields.stream()
+                .map(this::toClassField)
+                .toList();
+        final var classMethods = methods.stream()
+                .map(this::toClassMethod)
+                .toList();
+        final var declaration = new ClassDeclaration(name, classFields, classMethods);
+        ScopeHandler.setClassDeclaration(declaration);
         return NumberValue.ZERO;
     }
+
+    private ClassField toClassField(AssignmentExpression f) {
+        // TODO check only variable assignments
+        final String fieldName = ((VariableExpression) f.target).name;
+        return new ClassField(fieldName, f);
+    }
+
+    private ClassMethod toClassMethod(FunctionDefineStatement m) {
+        final var function = new UserDefinedFunction(m.arguments, m.body, m.getRange());
+        return new ClassMethod(m.name, function);
+    }
     
     @Override
     public void accept(Visitor visitor) {
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java
index b60c19c..09b3f99 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java
+++ b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java
@@ -50,7 +50,7 @@ public final class ContainerAccessExpression implements Node, Accessible {
             case Types.ARRAY -> ((ArrayValue) container).get(lastIndex);
             case Types.MAP -> ((MapValue) container).get(lastIndex);
             case Types.STRING -> ((StringValue) container).access(lastIndex);
-            case Types.CLASS -> ((ClassInstanceValue) container).access(lastIndex);
+            case Types.CLASS -> ((ClassInstance) container).access(lastIndex);
             default -> throw new TypeException("Array or map expected. Got " + Types.typeToString(container.type()));
         };
     }
@@ -62,7 +62,7 @@ public final class ContainerAccessExpression implements Node, Accessible {
         switch (container.type()) {
             case Types.ARRAY -> ((ArrayValue) container).set(lastIndex.asInt(), value);
             case Types.MAP -> ((MapValue) container).set(lastIndex, value);
-            case Types.CLASS -> ((ClassInstanceValue) container).set(lastIndex, value);
+            case Types.CLASS -> ((ClassInstance) container).set(lastIndex, value);
             default -> throw new TypeException("Array or map expected. Got " + container.type());
         }
         return value;
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/FunctionDefineStatement.java b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/FunctionDefineStatement.java
index f812756..0876802 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/FunctionDefineStatement.java
+++ b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/FunctionDefineStatement.java
@@ -48,8 +48,8 @@ public final class FunctionDefineStatement implements Statement, SourceLocation
 
     @Override
     public String toString() {
-        if (body instanceof ReturnStatement) {
-            return String.format("def %s%s = %s", name, arguments, ((ReturnStatement)body).expression);
+        if (body instanceof ReturnStatement rs) {
+            return String.format("def %s%s = %s", name, arguments, rs.expression);
         }
         return String.format("def %s%s %s", name, arguments, body);
     }
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ObjectCreationExpression.java b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ObjectCreationExpression.java
index 5204f3d..a19bbf6 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ObjectCreationExpression.java
+++ b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/ast/ObjectCreationExpression.java
@@ -29,41 +29,29 @@ public final class ObjectCreationExpression implements Node, SourceLocation {
     
     @Override
     public Value eval() {
-        final ClassDeclarationStatement cd = ClassDeclarations.get(className);
-        if (cd == null) {
-            // Is Instantiable?
-            if (ScopeHandler.isVariableOrConstantExists(className)) {
-                final Value variable = ScopeHandler.getVariableOrConstant(className);
-                if (variable instanceof Instantiable instantiable) {
-                    return instantiable.newInstance(ctorArgs());
-                }
+        final ClassDeclaration cd = ScopeHandler.getClassDeclaration(className);
+        if (cd != null) {
+            return cd.newInstance(constructorArgs());
+        }
+
+        // Is Instantiable?
+        if (ScopeHandler.isVariableOrConstantExists(className)) {
+            final Value variable = ScopeHandler.getVariableOrConstant(className);
+            if (variable instanceof Instantiable instantiable) {
+                return instantiable.newInstance(constructorArgs());
             }
-            throw new UnknownClassException(className, range);
         }
-        
-        // Create an instance and put evaluated fields with method declarations
-        final ClassInstanceValue instance = new ClassInstanceValue(className);
-        for (AssignmentExpression f : cd.fields) {
-            // TODO check only variable assignments
-            final String fieldName = ((VariableExpression) f.target).name;
-            instance.addField(fieldName, f.eval());
-        }
-        for (FunctionDefineStatement m : cd.methods) {
-            instance.addMethod(m.name, new ClassMethod(m.arguments, m.body, instance, m.getRange()));
-        }
-        
-        // Call a constructor
-        instance.callConstructor(ctorArgs());
-        return instance;
+        throw new UnknownClassException(className, range);
     }
-    
-    private Value[] ctorArgs() {
+
+    private Value[] constructorArgs() {
         final int argsSize = constructorArguments.size();
-        final Value[] ctorArgs = new Value[argsSize];
-        for (int i = 0; i < argsSize; i++) {
-            ctorArgs[i] = constructorArguments.get(i).eval();
+        final Value[] args = new Value[argsSize];
+        int i = 0;
+        for (Node argument : constructorArguments) {
+            args[i++] = argument.eval();
         }
-        return ctorArgs;
+        return args;
     }
     
     @Override
diff --git a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/optimization/OptimizationVisitor.java b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/optimization/OptimizationVisitor.java
index 4a42c70..fee1831 100644
--- a/ownlang-parser/src/main/java/com/annimon/ownlang/parser/optimization/OptimizationVisitor.java
+++ b/ownlang-parser/src/main/java/com/annimon/ownlang/parser/optimization/OptimizationVisitor.java
@@ -75,6 +75,7 @@ public abstract class OptimizationVisitor<T> implements ResultVisitor<Node, T> {
 
     @Override
     public Node visit(ClassDeclarationStatement s, T t) {
+        // TODO fields and methods
         return s;
     }