In a unique collaboration, Nature Publishing Group and Institute of Physics Publishing published the most extensive and comprehensive reference work in astronomy and astrophysics in both print and online formats. First published as a four volume print edition in 2001, the initial Web version went live in 2002, and contained the original print material and was rapidly supplemented with numerous updates and newly commissioned material. Since August 2004 the Encyclopedia is published solely by the Institute of Physics Publishing.This unique resource covers the entire field of astronomy and astrophysics and this online version includes the full text of over 2,750 articles, plus sophisticated search and retrieval functionality, links to the primary literature, and is frequently updated with new material. An active editorial team, headed by the Encyclopedia's editor-in-chief, Paul Murdin, oversees the continual commissioning, reviewing and loading of new and revised content.The Encyclopedia's authority is assured by editorial and advisory boards drawn from the world's foremost astronomers and astrophysicists. This first class resource will be an essential source of information for undergraduates, graduate students, researchers and seasoned professionals, as well as for committed amateurs, librarians and lay people wishing to consult the definitive astronomy and astrophysics reference work.