The Einstein equations have been a source of many interesting
problems in Physics, Analysis and Geometry. Despite the great deal of work
which has been devoted to them, with many success stories, several
important questions remain open. One of the them is a satisfactory theory
of isolated systems, such as stars, both from a perspective of the time
development of the space-time, as well as from the point of view of the
geometry induced on a space-like three surface. This talk will focus on the
former situation. More specifically, we shall discuss relativistic fluids
with and without viscosity, and prove a well-posedness result for the
Cauchy problem. The viscous case, in particular, is of significant interest
in light of recent developments in Astrophysics.