A collaboration between the Institute of Physics Publishing and Macmillan Reference Ltd, this work presents an extensive and comprehensive reference on astronomy and astrophysics. It has 800 contributors, over 2500 entries, is extensively cross-referenced - over 7500 links, and has extensive indexes - up to 12000 index entries. It should be of interest to undergraduates, graduate students, researchers and professionals, as well as to committed amateurs, librarians and lay people.