abstract: We shall introduce the technique of moving planes, relying on the invariance of an elliptic PDE by reflection with respect to a plane and the maximum principle, to get symmetry of solutions of this PDE. We shall give many examples, starting from the simplest and most geometric ones to more refined ones. We will also discuss the more recent method of moving spheres, based on the invariance of the equation with respect to an inversion. This second method permits to treat the case of systems of elliptic PDEs, for instance, and works in some cases where the moving planes technique is difficult or impossible to use.