  The content of Einstein's theory of gravitation is encoded in the properties
of the solutions to his field equations. There has been obtained a wealth of
information about these solutions in the ninety years the theory has been
around. It led to the prediction and the observation of physical phenomena
which confirm the important role of general relativity in physics. The
understanding of the domain of highly dynamical, strong field configurations
is, however, still quite limited. The gravitational wave experiments are likely
to provide soon observational data on phenomena which are not accessible by
other means. Further theoretical progress will require, however, new methods
for the analysis and the numerical calculation of the solutions to Einstein's
field equations on large scales and under general assumptions. We discuss some
of the problems involved, describe the status of the field and recent results,
and point out some open problems.
