In 1972, James Lovelock and Lynn Margulis began collaborating on the Gaia hypothesis. They suggested that over geological time, life on Earth has had a major role in both producing and regulating its own environment. Gaia is now an ecological and environmental worldview underpinning vital scientific and cultural debates over environmental issues. Their ideas have transformed the Earth and life sciences, as well as contemporary conceptions of nature. Their correspondence describes these crucial developments from the inside, showing how their partnership proved decisive for the development of the Gaia hypothesis. Clarke and Dutreuil provide historical background and explain the concepts and references introduced throughout the Lovelock-Margulis correspondence, while highlighting the major landmarks of their collaboration within the sequence of almost 300 letters written between 1970 and 2007. This book will be of interest to researchers in ecology, history of science, environmental history and climate change, and cultural science studies.