diff --git a/lang/fv.rkt b/lang/fv.rkt
new file mode 100644
index 0000000..212efad
--- /dev/null
+++ b/lang/fv.rkt
@@ -0,0 +1,21 @@
+#lang racket
+(require "ast.rkt")
+(provide fv)
+
+;; Expr -> [Listof Id]
+;; List all of the free variables in e
+(define (fv e)
+  (remove-duplicates (fv* e)))
+
+(define (fv* e)  
+  (match e
+    [(Var x)            (list x)]
+    [(Prim1 p e)        (fv* e)]
+    [(Prim2 p e1 e2)    (append (fv* e1) (fv* e2))]
+    [(Prim3 p e1 e2 e3) (append (fv* e1) (fv* e2) (fv* e3))]
+    [(If e1 e2 e3)      (append (fv* e1) (fv* e2) (fv* e3))]
+    [(Begin e1 e2)      (append (fv* e1) (fv* e2))]
+    [(Let x e1 e2)      (append (fv* e1) (remq* (list x) (fv* e2)))]
+    [(App e1 es)        (append (fv* e1) (append-map fv* es))]
+    [(Lam f xs e)       (remq* xs (fv* e))]
+    [_                  '()]))