This sixth volume in the ISSI Space Sciences Series is the outcome of a three-year-long process of carrying through the ISSI study project on `Source and Loss Processes of Magnetospheric Plasma'. The goal has been to produce a fully integrated book on the subject which gives an authoritative overview of all aspects of the topic in a well organized form, useful and readable both for active researchers in the field and for young scientists who are starting their research in space physics. In order to represent the full diversity of experience and perspective that exists in the science community, some fifty leading scientists from all over the world were invited to participate in the project and contribute to the text. All contributions are of high scientific standard, consistent in style, nomenclature, notations and format, and with good cross-referencing. This book is primarily of interest to researchers and graduate students in space physics and astrophysics as a textbook and handbook, but it will be a natural part of the library of all research groups involved in space matters.