The Geobiology option of GPS comprises scientists and students working at the interface between the biosphere and the physical Earth. We ask questions that span the range of geologic time, from the emergence and evolution of life on the earliest Earth to its occurrence in weird and wonderful extreme environments to its role in shaping modern biogeochemical cycles. These studies often blur the boundaries between geology, geochemistry, environmental science, microbiology, and molecular genetics. We offer both an undergraduate major and a graduate Ph.D. program. Our students and postdocs go on to do academic research in a variety of biological and Earth science fields, and to work in a variety of industries from pharmaceuticals to genetics to medicine. Caltech geobiologists explore such questions through a combination of work on modern organisms, reconstruction of phylogenetic histories, and direct examination of the geologic record.