The content of Einstein's theory is encoded in the properties
of the solutions to Einstein's field equations and the structure
of the solution manifold. In spite of its amazing successes the
theory still poses important mathematical and physical questions,
which mainly remained unsolved due to the peculiar properties of Einstein's
field equations. In the talk we will outline the present
state of our understanding, indicate the problems which are guiding research
in this field, discuss basic features of the field equations,
and present some recent results on the solutions in the large and their
qualitative behaviour.