The stars that comprise a halo around our Galaxy are intriguing; they have abundances of elements that suggest they are old. The properties of these "hot stars" and similar stars in other galaxies challenge the current, well-established theory of stellar evolution in many ways. In this book, experts contribute up-to-date and comprehensive reviews of the major topics in the field. They study the collective properties of these stars that provide important revelations for many areas of astrophysics--including how our Galaxy was formed. The contributors also reveal the exciting renaissance occurring in their studies owing to important new observations with the Hubble Space Telescope and ASTRO1 and to techniques recently developed for ground-based observations. Advances in computers have also recently led to a far more detailed and complete theoretical understanding of stellar evolution. This book will be of interest to researchers and graduate students in astronomy and astrophysics.