diff --git a/doc/site/core/fn.markdown b/doc/site/core/fn.markdown
index 5bd2358a..1491a881 100644
--- a/doc/site/core/fn.markdown
+++ b/doc/site/core/fn.markdown
@@ -18,6 +18,14 @@ you create a "bare" function when you don't want to immediately pass it as a
 
 It is a runtime error if `block` is not a function.
 
+### **arity**
+
+The number of arguments the function requires.
+
+    :::dart
+    IO.print(new Fn {}.arity)             // 0.
+    IO.print(new Fn {|a, b, c| a }.arity) // 3.
+
 ### **call**(args...)
 
 **TODO**
diff --git a/doc/site/functions.markdown b/doc/site/functions.markdown
index d828802d..7a4fe7f1 100644
--- a/doc/site/functions.markdown
+++ b/doc/site/functions.markdown
@@ -115,8 +115,8 @@ Here we're passing a function to `greet` that takes two parameters, `first` and
       }
     }
 
-It's an error to call a function with fewer or more arguments than its
-parameter list expects.
+It's an error to call a function with fewer arguments than its parameter list
+expects. If you pass too *many* arguments, the extras are ignored.
 
 ## Returning values
 
diff --git a/include/wren.h b/include/wren.h
index 787b5266..5739b930 100644
--- a/include/wren.h
+++ b/include/wren.h
@@ -97,11 +97,11 @@ WrenInterpretResult wrenInterpret(WrenVM* vm, const char* sourcePath,
 // global class named [className] to bind the method to. If not found, it will
 // be created automatically.
 //
-// Defines a method on that class named [methodName] accepting [numParams]
+// Defines a method on that class named [methodName] accepting [arity]
 // parameters. If a method already exists with that name and arity, it will be
 // replaced. When invoked, the method will call [method].
 void wrenDefineMethod(WrenVM* vm, const char* className,
-                      const char* methodName, int numParams,
+                      const char* methodName, int arity,
                       WrenForeignMethodFn method);
 
 // Defines a static foreign method implemented by the host application. Looks
@@ -109,10 +109,10 @@ void wrenDefineMethod(WrenVM* vm, const char* className,
 // will be created automatically.
 //
 // Defines a static method on that class named [methodName] accepting
-// [numParams] parameters. If a method already exists with that name and arity,
+// [arity] parameters. If a method already exists with that name and arity,
 // it will be replaced. When invoked, the method will call [method].
 void wrenDefineStaticMethod(WrenVM* vm, const char* className,
-                            const char* methodName, int numParams,
+                            const char* methodName, int arity,
                             WrenForeignMethodFn method);
 
 // The following functions read one of the arguments passed to a foreign call.
diff --git a/src/wren_core.c b/src/wren_core.c
index d1601b39..b551f701 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -489,23 +489,6 @@ DEF_NATIVE(fiber_yield1)
   return PRIM_RUN_FIBER;
 }
 
-static PrimitiveResult callFunction(WrenVM* vm, Value* args, int numArgs)
-{
-  ObjFn* fn;
-  if (IS_CLOSURE(args[0]))
-  {
-    fn = AS_CLOSURE(args[0])->fn;
-  }
-  else
-  {
-    fn = AS_FN(args[0]);
-  }
-
-  if (numArgs < fn->numParams) RETURN_ERROR("Function expects more arguments.");
-
-  return PRIM_CALL;
-}
-
 DEF_NATIVE(fn_instantiate)
 {
   // Return the Fn class itself. When we then call "new" on it, it will
@@ -521,6 +504,28 @@ DEF_NATIVE(fn_new)
   RETURN_VAL(args[1]);
 }
 
+DEF_NATIVE(fn_arity)
+{
+  RETURN_NUM(AS_FN(args[0])->arity);
+}
+
+static PrimitiveResult callFunction(WrenVM* vm, Value* args, int numArgs)
+{
+  ObjFn* fn;
+  if (IS_CLOSURE(args[0]))
+  {
+    fn = AS_CLOSURE(args[0])->fn;
+  }
+  else
+  {
+    fn = AS_FN(args[0]);
+  }
+
+  if (numArgs < fn->arity) RETURN_ERROR("Function expects more arguments.");
+
+  return PRIM_CALL;
+}
+
 DEF_NATIVE(fn_call0) { return callFunction(vm, args, 0); }
 DEF_NATIVE(fn_call1) { return callFunction(vm, args, 1); }
 DEF_NATIVE(fn_call2) { return callFunction(vm, args, 2); }
@@ -1245,6 +1250,7 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->fnClass->obj.classObj, " instantiate", fn_instantiate);
   NATIVE(vm->fnClass->obj.classObj, "new ", fn_new);
 
+  NATIVE(vm->fnClass, "arity", fn_arity);
   NATIVE(vm->fnClass, "call", fn_call0);
   NATIVE(vm->fnClass, "call ", fn_call1);
   NATIVE(vm->fnClass, "call  ", fn_call2);
@@ -1262,7 +1268,6 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->fnClass, "call              ", fn_call14);
   NATIVE(vm->fnClass, "call               ", fn_call15);
   NATIVE(vm->fnClass, "call                ", fn_call16);
-  // TODO: "arity" getter.
   NATIVE(vm->fnClass, "toString", fn_toString);
 
   vm->nullClass = defineClass(vm, "Null");
diff --git a/src/wren_value.c b/src/wren_value.c
index bcab27a5..8595e534 100644
--- a/src/wren_value.c
+++ b/src/wren_value.c
@@ -160,7 +160,7 @@ ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn)
 }
 
 ObjFn* wrenNewFunction(WrenVM* vm, Value* constants, int numConstants,
-                       int numUpvalues, int numParams,
+                       int numUpvalues, int arity,
                        uint8_t* bytecode, int bytecodeLength,
                        ObjString* debugSourcePath,
                        const char* debugName, int debugNameLength,
@@ -200,7 +200,7 @@ ObjFn* wrenNewFunction(WrenVM* vm, Value* constants, int numConstants,
   fn->constants = copiedConstants;
   fn->numUpvalues = numUpvalues;
   fn->numConstants = numConstants;
-  fn->numParams = numParams;
+  fn->arity = arity;
   fn->bytecodeLength = bytecodeLength;
   fn->debug = debug;
 
diff --git a/src/wren_value.h b/src/wren_value.h
index a29a0290..16c0f964 100644
--- a/src/wren_value.h
+++ b/src/wren_value.h
@@ -246,7 +246,7 @@ typedef struct
   // The number of parameters this function expects. Used to ensure that .call
   // handles a mismatch between number of parameters and arguments. This will
   // only be set for fns, and not ObjFns that represent methods or scripts.
-  int numParams;
+  int arity;
   FnDebug* debug;
 } ObjFn;
 
@@ -571,7 +571,7 @@ ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn);
 // function will take over ownership of [bytecode] and [sourceLines]. It will
 // copy [constants] into its own array.
 ObjFn* wrenNewFunction(WrenVM* vm, Value* constants, int numConstants,
-                       int numUpvalues, int numParams,
+                       int numUpvalues, int arity,
                        uint8_t* bytecode, int bytecodeLength,
                        ObjString* debugSourcePath,
                        const char* debugName, int debugNameLength,
diff --git a/test/function/arity.wren b/test/function/arity.wren
new file mode 100644
index 00000000..9e6047e4
--- /dev/null
+++ b/test/function/arity.wren
@@ -0,0 +1,5 @@
+IO.print(new Fn {}.arity) // expect: 0
+IO.print(new Fn {|a| a}.arity) // expect: 1
+IO.print(new Fn {|a, b| a}.arity) // expect: 2
+IO.print(new Fn {|a, b, c| a}.arity) // expect: 3
+IO.print(new Fn {|a, b, c, d| a}.arity) // expect: 4