This book gives an account of the proceedings of the International Astronomical Union Colloquium 115: High Resolution X-Ray Spectroscopy of Cosmic Plasmas. This was the first IAU meeting dedicated to high resolution X-ray spectroscopy of objects outside the solar system. A broad range of objects and astrophysical conditions are discussed. Results from the first generation of satellites with spectroscopic capability, i.e. the Einstein Observatory, EXOSAT, and Tenma, are reviewed from a perspective of a more precise interpretation allowed by improved theoretical models and plasma diagnostics. Laboratory and solar X-ray results that model or are relevant to conditions found in cosmic X-ray sources are also presented. The colloquium presents a forum for discussion of scientific objectives of new international missions in high resolution X-ray spectroscopy.