The stars that form 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. Studying the collective properties of these stars provides important input to many areas of astrophysics - including the formation of our Galaxy, the late phases of stellar evolution and the stellar populations of other galaxies. The study of hot stars in the halo of our Galaxy is undergoing an exciting renaissance 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. A conference was held in Union College, New York, to gather experts in the field and this volume draws together their articles to provide an up-to-date and comprehensive review for graduate students and researchers.