In vertebrates, β cells are aggregated in the form of pancreatic islets. Within these islets, communication between β cells inhibits basal insulin secretion and enhances glucose-stimulated insulin secretion, thus contributing to glucose homeostasis during fasting and feeding. In the search for the underlying molecular mechanism, we have discovered that β cells communicate via ephrin-As and EphAs. We provide evidence that ephrin-A5 is required for glucose-stimulated insulin secretion. We further show that EphA-ephrin-A-mediated β cell communication is bidirectional: EphA forward signaling inhibits insulin secretion, whereas ephrin-A reverse signaling stimulates insulin secretion. EphA forward signaling is downregulated in response to glucose, which indicates that, under basal conditions, β cells use EphA forward signaling to suppress insulin secretion and that, under stimulatory conditions, they shift to ephrin-A reverse signaling to enhance insulin secretion. Thus, we explain how β cell communication in pancreatic islets conversely affects basal and glucose-stimulated insulin secretion to improve glucose homeostasis.