  The evolution of a generally covariant theory is under-determined. One
hundred years ago such dynamics had never before been considered; its
ramifications were perplexing, its future important role for all the
fundamental interactions under the name gauge principle could not be foreseen.
We recount some history regarding Einstein, Hilbert, Klein and Noether and the
novel features of gravitational energy that led to Noether's two theorems.
Under-determined evolution is best revealed in the Hamiltonian formulation. We
developed a covariant Hamiltonian formulation. The Hamiltonian boundary term
gives covariant expressions for the quasi-local energy, momentum and angular
momentum. Gravity can be considered as a gauge theory of the local Poincar\'e
group. The dynamical potentials of the Poincar\'e gauge theory of gravity are
the frame and the connection. The spacetime geometry has in general both
curvature and torsion. Torsion naturally couples to spin; it could have a
significant magnitude and yet not be noticed, except on a cosmological scale
where it could have significant effects.
