The lecture begins with a question-and-answer session about black holes. Topics include the extent to which we are sure black holes exist in the center of all galaxies, how massive they are, and how we can observe them. The lecture then turns to strong-field relativity: relativistic effects that are unrelated to Newtonian theory. The possibility of testing predictions of the existence of black holes is discussed in the context of strong-field relativity. One way we might learn about black holes is through observation of the orbit of the companion star in an X-ray binary star system. Through this we can estimate the mass of the compact object. The lecture ends with an explanation of how astronomers find black holes, and how Professor Bailyn was able to discover one himself.