The X-ray transients currently provide the best evidence for the existence of stellar mass black holes. These objects are interacting binary systems which spend most of their time in a state where relatively little accretion onto the compact object takes place. There are, however, sudden episodes of relatively high accretion on the compact object which lead to an abrupt and dramatic increase in the X-ray and optical luminosity. During the periods of low accretion, optical studies of the companion star lead to a dynamical mass estimates for both components. In 16 cases, the mass of the compact object in an X-ray transient has been shown to exceed the maximum mass of a stable neutron star (about 3 solar masses), which leads to the conclusion that these objects are black holes. In this colloquium I will review the basic properties of these black hole systems. I will also discuss the latest results on observations of the supermassive black hole that resides at the center of the Milky Way Galaxy.