Abstract: Due to the absence of local gravitational degrees of freedom, Einstein's theory of gravity in (2+1) dimensions can be formulated as a Chern-Simons gauge theory. The Chern-Simons formulation of (2+1)-dimensional gravity provides an efficient parametrisation of phase space and Poisson structure, which serves as a starting point for quantisation, as well as a complete set of gauge invariant Wilson loop observables. Its drawback is that it obscures the underlying spacetime geometry and thereby complicates the physical interpretation of the theory. In my talk I relate the geometrical and the Chern-Simons description of vacuum spacetimes of general genus and with general cosmological constant. I discuss how the geometry of the spacetime can be recovered from the variables parametrising the phase space in the Chern-Simons formalism and how changes of geometry manifest themselves as transformations on the phase space. I show that the two basic transformations which change the geometry of the spacetime, infinitesimal Dehn twists and grafting along a closed, simple geodesic, are generated via the Poisson bracket by the two associated canonical Wilson loop observables. By introducing a description in which the cosmological constant plays the role of a deformation parameter, I demonstrate that these two transformations are closely related and that grafting can be viewed as an infinitesimal Dehn twist (earthquake) with a formal parameter whose square is minus the cosmological constant. References: C. Meusburger, Geometrical (2+1)-gravity and the Chern-Simons formulation: Grafting, Dehn twists, Wilson loop observables and the cosmological constant, Commun. Math. Phys. 273 (2007) 705–754. C. Meusburger, Grafting and Poisson structure in (2+1)-gravity with vanishing cosmological constant, Commun. Math. Phys. 266 (2006) 735--775,