The first edition of this book, published by University Press of New England in 1986, sold over 2500 copies, and was received as the best introductory overview of this broad field. Quite a lot has happened in the field of symbiosis in the past 10 years, especially concerning molecular mechanisms. Ahmadjian and Paracer have thoroughly updated their book, addressing advances in the field and the emergence of fields such as cellular microbiology, immunoparasitology, and endocytobiology, which have revealed new aspects of symbiosis. It is the only book to cover all aspects of symbiosis at an introductory level.