We use in vivo models to study how insulin-producing cells emerge from progenitor cells during fetal development. Specifically, how are cell fate decisions made when progenitor cells divide? Which transcription factors are important to specify beta cell identity? How do progenitor cells exit the cell cycle to undergo differentiation? We apply our understanding of the principles of pancreatic development to thinking about the factors necessary to more efficiently and effectively differentiate stem cells into beta cells or to reprogram other cell types to become beta cells.