  Proof that black holes exist will likely require confirmation of the
existence of event horizons. The common assumption that the mere existence of
large compact masses proves the case for black holes is an unwarranted
extrapolation of General Relativity into a strong-field regime where it has not
been adequately tested. Neither the large compact masses of galactic nuclei nor
the massive compact objects of stellar mass in the x-ray binaries prove the
existence of black holes. In contrast to the case for galactic nuclei, we have
the necessary tools for obtaining either proof or disproof of event horizons in
the x-ray binaries. Observations of kHz QPOs may decide the event horizon issue
very quickly. If not, we can still obtain proof by comparing predictions of
gravity theories that differ primarily by the presence or absence of an event
horizon. Detailed analysis of models of x-ray binaries would then decide the
issue.
