  This thesis is devoted to the group-theoretic aspects of three-dimensional
quantum gravity on Anti-de Sitter and Minkowskian backgrounds. In particular we
describe the relation between unitary representations of asymptotic symmetry
groups and gravitational perturbations around a space-time metric. In the
asymptotically flat case this leads to "BMS particles", representing standard
relativistic particles dressed with gravitational degrees of freedom accounted
for by coadjoint orbits of the Virasoro group. Their thermodynamics are
described by BMS characters, which coincide with gravitational one-loop
partition functions. We also extend these considerations to higher-spin
theories and supergravity.
