This paper discusses some recent results on the related problems of positivity of mass and the existence of maximal spacelike hypersurfaces
for asymptotically flat space-times. If
k
denotes the second fundamental form of
, then
is called
maximal if
trk = 0
, has constant mean extrinsic curvature if
trk = const
on
, and is a moment of time-symmetry if
k = 0
.
We will give a certain amount of background material in order to keep the exposition self-contained. However, we so assume familiarity with the ADM formalism.