  We discuss new observations of X-ray novae which provide strong evidence that
black holes have event horizons. Optical observations of 13 X-ray novae
indicate that these binary stars contain collapsed objects too heavy to be
stable neutron stars. The objects have been identified as black hole
candidates. X-ray observations of several of these X-ray novae in quiescence
with the Chandra X-ray Observatory show that the systems are approximately 100
times fainter than nearly identical X-ray novae containing neutron stars. The
advection-dominated accretion flow model provides a natural explanation for the
difference. In this model, the accreting gas reaches the accretor at the center
with a large amount of thermal energy. If the accretor is a black hole, the
thermal energy will disappear through the event horizon, and the object will be
very dim. If the accretor is a neutron star or any other object with a surface,
the energy will be radiated from the surface, and the object will be bright. We
discuss alternate interpretations of the data that eliminate the need for
advection-dominated accretion. Most of these alternatives still require an
event horizon to explain the unusually low X-ray luminosities of the black hole
candidates. Some of the alternatives are also inconsistent with observations.
