  Nonlinear realizations of spacetime groups are presented as a versatile
mathematical tool providing a common foundation for quite different
formulations of gauge theories of gravity. We apply nonlinear realizations in
particular to both the Poincar\'e and the affine group in order to develop
Poincar\'e gauge theory (PGT) and metric-affine gravity (MAG) respectively.
Regarding PGT, two alternative nonlinear treatments of the Poincar\'e group are
developed, one of them being suitable to deal with the Lagrangian and the other
one with the Hamiltonian version of the same gauge theory. We argue that our
Hamiltonian approach to PGT is closely related to Ashtekar's approach to
gravity. On the other hand, a brief survey on MAG clarifies the role played by
the metric--affine metric tensor as a Goldsone field. All gravitational
quantities in fact --the metric as much as the coframes and connections-- are
shown to acquire a simple gauge--theoretical interpretation in the nonlinear
framework.

General Relativity and Quantum Cosmology