  The treatment of exact conservation laws in Lagrangian gauge theories
constitutes the main axis of the first part of the thesis. The formalism is
developed as a self-consistent theory but is inspired by earlier works, mainly
by cohomological results, covariant phase space methods and by the Hamiltonian
formalism. The thermodynamical properties of black holes, especially the first
law, are studied in a general geometrical setting and are worked out for
several black objects: black holes, strings and rings. Also, the geometrical
and thermodynamical properties of a new family of black holes with closed
timelike curves in three dimensions are described.
  The second part of the thesis is the natural generalization of the first part
to asymptotic analyses. We start with a general construction of covariant phase
spaces admitting asymptotically conserved charges. The representation of the
asymptotic symmetry algebra by a covariant Poisson bracket among the conserved
charges is then defined and is shown to admit generically central extensions.
The asymptotic structures of three three-dimensional spacetimes are then
studied in detail and the consequences for quantum gravity in three dimensions
are discussed.
