The Stokes waves are periodic symmetric steady water waves travelling at a constant speed, which plays a fundamental role in the study of water waves. It was observed by Benjamin and Feir in 1967 that the Stokes waves are subject to the modulational instability. However, the rigorous mathematical proof was missing for a long time. In this talk, we will discuss how to prove the nonlinear modulational instability of the Stokes waves in the context of 2d full water waves. This is joint work with Gong Chen.