Pancreatic β-cells are responsible for producing all of the insulin required by an organism to maintain glucose homeostasis. Defects in development, maintenance, or expansion of β-cell mass can result in impaired glucose metabolism and diabetes. Thus, identifying the molecular regulators of these processes may provide new therapeutic targets for diabetes. Additionally, understanding the processes of β-cell differentiation and proliferation may allow for in vitro cultivation of β-cells in sufficient amounts to be transplanted into patients with diabetes. This review addresses many of the transcription factors and signaling pathways that play a role in early pancreatic development and endocrine cell (specifically β-cell) differentiation, conditions that influence β-cell mass development and molecular regulators of β-cell proliferation and apoptosis that are responsible for maintaining and expanding β-cell mass.