diff --git a/lval/environment.c b/lval/environment.c
index 86a5b22..975387a 100644
--- a/lval/environment.c
+++ b/lval/environment.c
@@ -93,4 +93,32 @@ void lenv_add_builtins(lenv* e) {
     lenv_add_builtin(e, "%", builtin_mod);
     lenv_add_builtin(e, "min", builtin_min);
     lenv_add_builtin(e, "max", builtin_max);
+    
+    lenv_add_builtin(e, "def", builtin_def);
+}
+
+lval* builtin_def(lenv* e, lval* a) {
+    LASSERT(a, a->cell[0]->type == LVAL_QEXPR,
+        "Function 'def' passed incorrect type")
+
+    // First argument is the symbol list
+    lval* syms = a->cell[0];
+
+    // Ensure all elements of the first list are symbols
+    for (int i = 0; i < syms->count; i++) {
+        LASSERT(a, syms->cell[i]->type == LVAL_SYM,
+            "Function 'def' cannot define non-symbol")
+    }
+
+    // Check correct number of symbols and values
+    LASSERT(a, syms->count == a->count - 1,
+        "Function 'def' cannot define incorrect number of values to symbols")
+
+    // Assign copies of values to symbols
+    for (int i = 0; i < syms->count; i++) {
+        lenv_put(e, syms->cell[i], a->cell[i + 1]);
+    }
+
+    lval_del(a);
+    return lval_sexpr();
 }
\ No newline at end of file
diff --git a/lval/environment.h b/lval/environment.h
index 6845af3..6444731 100644
--- a/lval/environment.h
+++ b/lval/environment.h
@@ -21,5 +21,7 @@ lval* lval_fun(lbuiltin func);
 void lenv_add_builtin(lenv* e, char* name, lbuiltin func);
 void lenv_add_builtins(lenv* e);
 
+lval* builtin_def(lenv* e, lval* a);
+
 
 #endif
diff --git a/prompt.c b/prompt.c
index f56c269..9aa616e 100644
--- a/prompt.c
+++ b/prompt.c
@@ -96,6 +96,8 @@ int main (int argc, char** argv) {
 		free(input);
 	}
 
+	lenv_del(e);
+
 	mpc_cleanup(8, Number, Long, Double, Symbol, Sexpr, Qexpr, Expr, Lispy);
 	return 0;
 }