Geometrically the phase space of a mechanical system involves the cotangent bundle of the configuration space. The phase space of a relativistic field theory is infinite dimensional and can be endowed with a symplectic structure defined in a perfectly covariant manner that is very useful for discussing symmetries and conserved quantities of the system. In general relativity the symplectic structure takes the Darboux form, and it is shown in this work that the presence of a cosmological constant does not change this conclusion. For space-times that admit timelike Killing vectors the formalism can be used to define mass in general relativity, and it is known that, for asymptotically flat black holes, this mass is identical to the usual Arnowitt-Desner-Misner mass while for asymptotically anti–de Sitter Kerr metrics it is the same as the Henneaux-Teitelboim mass. We show that the same formalism can also be used to derive the Brown-York mass and the Bondi mass for stationary space times, in particular the Brown-York mass has a natural interpretation in terms of differential forms on the space of solutions of the theory.