This paper compares two alternative theories of Aggregate supply, both with a "New Keynesian Flavor". The first assumes that prices are rigid due to thè existence of menu costs of thè kind advanced by Mankiw [38] and Akerlof and Yellen [2j. The second derives price stickiness endogenously as one equilibrium in an economy with multiple equilibria. In both cases we show that thè Ball-Romer concepì of real rigidities is essential to explain why monetary policy has real persistent effects.