This book contains the Proceedings of the Summer Study entitled Particle and Nuclear Astrophysics and Cosmology in the Next Millennium, held in July 1994 in Snowmass, Colorado. This two week-long intensive workshop was jointly sponsored by three Divisions of the American Physical Society: the Division of Astrophysics, the Division of Nuclear Physics, and the Division of Particles and Fields. The principal goal of the Summer Study was to examine the opportunities that are opening up in this cross-disciplinary field and help provide a critical evaluation of the future course of this emerging discipline. The Summer Study's title aptly conveyed the necessary joining of forces in the different disciplines needed for future progress. Snowmass 1994 represented the first time that members of the high energy and nuclear physics communities met with the community of astronomers, astrophysicists and cosmologists with the explicit purpose of exploring jointly areas of overlap and plotting a common future. This experiment proved to be both popular and successful, with nearly 450 participants attending the Summer Study.