Lynn Margulis and Dorion Sagan here present their fourth book as a writing team, a collection of their essays on Gaia theory, symbiosis, individuality, and the way science is practiced nowadays. Lynn Margulis is most famous for her now-widely-accepted proposition that the cells of higher plants and animals are not individuals but symbiotic unions of more primitive cells. In these essays, perhaps more clearly than in any of Margulis and Sagan's previous books, we can see how her seemingly disparate interests combine into a coherent and very provocative scientific world-view about the tendency of life to form complex communities.