In this article we explain the well-posedness of the initial value problem for water waves, and the shallow water and long wave approximations for water waves. Especially, we explain that the solution of the full water wave problem can be approximated by the solution of the shallow water equations, the Green–Naghdi equations, the KdV equation, the Kawahara equation, the forced KdV equation, and the Benjamin–Ono equation in some scaling regimes.