Peter and Rosemary Grant have been studying Darwin’s finches on the Galápagos islands since 1973. The fieldwork is designed to understand the causes of an adaptive radiation. It combines analyses of archipelago-wide patterns of evolution with detailed investigations on two islands, Genovesa and Daphne. By integrating ecology, behavior and genetics, they have revealed how species are reproductively and ecologically isolated, and the causes and evolutionary consequences of rare episodic hybridization. Two mechanisms of speciation have been identified. In collaboration with other investigators, they have estimate phylogenetic relationships among the species of finches and their relatives, identified molecular mechanisms involved in the development of beaks and provided genomic details of evolution in contemporary as well as historical time. The research has been published in many research papers and three books, most recently “How and Why Species Multiply” (2008) and “40 years of Evolution” (2014).