abstract: In the first lecture, I will decribe the Seiberg-Witten invariant introduced by Kronheimer and Mrowka for 4-manifold M with a contact boundary Y. In a second lecture I shall present my work with Tom Mrowka. If the Seiberg-Witten invariant is non-vanishing we say that the contact boundary is monopole fillable. Then, there is a strong interplay between the 3-dimensional contact geometry of Y and the 4-dimensional geometry of the filling. For example, we show that the Legendrian knots in Y are constrained by the Bennequin inequality.