Observations of distant supernovae have provided startling evidence that the expansion of the Universe may be accelerating, rather than decelerating. If this result is verified by future studies, it has profound implications for cosmology. The reliability of this finding and its implications for both the study of supernovae and cosmology are the subject of this exciting volume. Based on a conference at the University of Chicago, this timely volume, originally published in 2000, presents articles by leading experts on the theory of Type Ia supernovae, observational astronomy, and cosmology. It examines the observational data, the nature of the likely progenitor binary systems, the outburst mechanisms of Type Ia supernovae events, and the cosmological implications. This is a unique and wide-ranging review of one of the most dramatic and controversial results in astronomy in recent decades. It makes fascinating reading for all researchers and graduate students.