abstract: Given a surface M immersed in a Riemannian manifold (M,g) of dimension three, the Willmore functional I is defined as I(M)=\intM H2 dS where H is the mean curvature and dS the area form given by the immersion. The critical points of I are called Willmore surfaces and the aim of the seminar is to study the existence of such surfaces. The topic is classical and has many applications (general relativity, biology, elasticity theory...); after an introduction about the employed method (it is performed a finite dimensional reduction) we will study the functional in a perturbative setting.