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We discuss new observations of X-ray novae which provide strong evidence thatblack holes have event horizons. Optical observations of 13 X-ray novaeindicate that these binary stars contain collapsed objects too heavy to bestable neutron stars. The objects have been identified as black holecandidates. X-ray observations of several of these X-ray novae in quiescencewith the Chandra X-ray Observatory show that the systems are approximately 100times fainter than nearly identical X-ray novae containing neutron stars. Theadvection-dominated accretion flow model provides a natural explanation for thedifference. In this model, the accreting gas reaches the accretor at the centerwith a large amount of thermal energy. If the accretor is a black hole, thethermal energy will disappear through the event horizon, and the object will bevery 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. Wediscuss alternate interpretations of the data that eliminate the need foradvection-dominated accretion. Most of these alternatives still require anevent horizon to explain the unusually low X-ray luminosities of the black holecandidates. Some of the alternatives are also inconsistent with observations.Comment: Invited review, to appear in Proc. IX Marcel Grossmann Meeting, eds.  V. Gurzadyan, R. Jantzen and R. Ruffini, Singapore: World Scientific; 21  pages, 2 postscript figures, uses mprocl.sty (included); Table 2, Figs. 1, 2,  and Sec. 5 have been update

X-RAY NOVAE AND THE EVIDENCE FOR BLACK HOLE EVENT HORIZONS