diff --git a/src/com/craftinginterpreters/lox/Expr.java b/src/com/craftinginterpreters/lox/Expr.java
new file mode 100644
index 0000000..a59ebe4
--- /dev/null
+++ b/src/com/craftinginterpreters/lox/Expr.java
@@ -0,0 +1,15 @@
+package com.craftinginterpreters.lox;
+
+abstract class Expr {
+    static class Binary extends Expr {
+        Binary(Expr left, Token operator, Expr right) {
+            this.left = left;
+            this.operator = operator;
+            this.right = right;
+        }
+
+        final Expr left;
+        final Token operator;
+        final Expr right;
+    }
+}
diff --git a/src/com/craftinginterpreters/tool/GenerateAst.java b/src/com/craftinginterpreters/tool/GenerateAst.java
new file mode 100644
index 0000000..de85393
--- /dev/null
+++ b/src/com/craftinginterpreters/tool/GenerateAst.java
@@ -0,0 +1,69 @@
+package com.craftinginterpreters.tool;
+
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.util.Arrays;
+import java.util.List;
+
+public class GenerateAst {
+    public static void main(String[] args) throws IOException {
+        if (args.length != 1) {
+            System.err.println("Usage: generate_ast <output_directory>");
+            System.exit(64);
+        }
+        String outputDir = args[0];
+        defineAst(outputDir, "Expr", Arrays.asList(
+                "Binary   : Expr left, Token operator, Expr right",
+                "Grouping : Expr expression",
+                "Literal  : Object value",
+                "Unary    : Token operator, Expr right"
+        ));
+    }
+
+    private static void defineAst(String outputDir, String baseName, List<String> types)        throws IOException {
+        String path = outputDir + "/" + baseName + ".java";
+        PrintWriter writer = new PrintWriter(path, "UTF-8");
+
+        writer.println("package com.craftinginterpreters.lox;");
+        writer.println();
+        writer.println("import java.util.list;");
+        writer.println();
+        writer.println("abstract class " + baseName + " {");
+
+        // The AST classes
+        for (String type : types) {
+            String className = type.split(":")[0].trim();
+            String fields = type.split(":")[1].trim();
+            defineType(writer, baseName, className, fields);
+        }
+
+        writer.println("}");
+        writer.close();
+
+    }
+
+    private static void defineType(PrintWriter writer, String baseName, String className, String fieldList) {
+        writer.println("  static class " + className + " extends " + baseName + " {");
+
+        // Constructor
+        writer.println("    " + className + "(" + fieldList + ") {");
+
+        // Store parameters in fields
+        String []fields = fieldList.split(",");
+        for (String field : fields) {
+            String name = field.split(" ")[1];
+            writer.println("    this." + name + " = " + name + ";" );
+        }
+
+        writer.println("    }");
+
+        // Fields
+        writer.println();
+        for (String field : fields) {
+            writer.println("    final " + field + " = " + field + ";");
+        }
+
+        writer.println("  }");
+
+    }
+}