In this talk, we will present a new approach to study the well-posedness theory of the Prandtl equation in Sobolev spaces by using a direct energy method under a monotonicity condition on the tangential velocity field instead of using the Crocco transformation. Precisely, we firstly investigate the linearized Prandtl equation in some weighted Sobolev spaces when the tangential velocity of the  background state is  monotonic in the normal variable. Then to cope with the loss of regularity of the perturbation with respect to the background state due to the degeneracy of the equation,  we apply  the Nash-Moser-Hormander iteration to obtain a well-posedness theory of classical solutions to the nonlinear Prandtl equation  when the initial data is a small perturbation of a monotonic shear flow. We will mainly present the completed joint work with R. Alexandre, Y.-G. Wang and C.-J. Xu on the 2D problem, then briefly mention the current progress on 3D with C.-J. Liu, Y.-G. Wang and F. Yu.