diff --git a/src/com/annimon/everlastingsummer/Parser.java b/src/com/annimon/everlastingsummer/Parser.java
index cfaf965..f5fd54f 100644
--- a/src/com/annimon/everlastingsummer/Parser.java
+++ b/src/com/annimon/everlastingsummer/Parser.java
@@ -226,6 +226,20 @@ public final class Parser {
                 Variables.setVariable(token.getText(), expression().eval());
                 return false;
             }
+            if (lookMatch(1, TokenType.EQ) && match(TokenType.PLUS)) {
+                // variable += expression
+                consume(TokenType.EQ);
+                final double varValue = Variables.getVariable(token.getText());
+                Variables.setVariable(token.getText(), varValue + expression().eval());
+                return false;
+            }
+            if (lookMatch(1, TokenType.EQ) && match(TokenType.MINUS)) {
+                // variable -= expression
+                consume(TokenType.EQ);
+                final double varValue = Variables.getVariable(token.getText());
+                Variables.setVariable(token.getText(), varValue - expression().eval());
+                return false;
+            }
         }
         
         return false;