  This article is based on a pair of lectures given at the 2005 SLAC Summer
Institute. Our goal is to motivate why most physicists and astrophysicists
accept the hypothesis that the most massive, compact objects seen in many
astrophysical systems are described by the black hole solutions of general
relativity. We describe the nature of the most important black hole solutions,
the Schwarzschild and the Kerr solutions. We discuss gravitational collapse and
stability in order to motivate why such objects are the most likely outcome of
realistic astrophysical collapse processes. Finally, we discuss some of the
observations which -- so far at least -- are totally consistent with this
viewpoint, and describe planned tests and observations which have the potential
to falsify the black hole hypothesis, or sharpen still further the consistency
of data with theory.
