Two alternative theories of aggregate supply, both with a New Keynesian “flavor,” are compared. The first assumes that prices are rigid due to the existence of menu costs. The second derives price stickiness endogenously as one equilibrium in an economy with multiple equilibria. In both cases I show that the Ball–Romer concept of real rigidities is essential to explain why monetary policy has real persistent effects. I argue that dynamic menu cost models are determinate because they make special assumptions about the way that money enters the economy. For example, most authors assume either a cash-in-advance constraint or that money enters separably into utility or production functions. Once one moves beyond these special cases, menu cost models that display real rigidity are also likely to display indeterminacy.