  The classical notion of center of mass for an isolated system in general
relativity is derived from the Hamiltonian formulation and represented by a
flux integral at infinity. In contrast to mass and linear momentum which are
well-defined for asymptotically flat manifolds, center of mass and angular
momentum seem less well-understood, mainly because they appear as the lower
order terms in the expansion of the data than those which determine mass and
linear momentum. This article summarizes some of the recent developments
concerning center of mass and its geometric interpretation using the constant
mean curvature foliation near infinity. Several equivalent notions of center of
mass are also discussed.
