abstract: The goal of this course is to sketch the proof of a recent result, obtained in collaboration with Thomas Alazard and Nicolas Burq, which concerns the local existence of a solution for the Cauchy problem for the gravity water wave system in any dimension, when the data belong to a Sobolev space of low regularity. This course can be followed by phd students. Here are the main sections which will be developed. 1. Description of the equations. The Craig-Sulem-Zaharov formulation.The main result. 2. The Dirichlet- Neumann operator and its paralinearization. 3. The reformulation of the equations and the energy estimates. 4. End of the proof. 5. Description of several extensions and open problems.