Insulinomas are rare neuroendocrine tumors of pancreatic islet cells that retain the ability to produce and secrete insulin. In contrast to normally differentiated β-cells, insulinoma cells continue to secrete insulin and proinsulin at low blood glucose. This deregulated insulin secretion manifests clinically as fasting hypoglycemia. The molecular pathways that characterize normal insulin secretion and β-cell growth are reviewed and contrasted to the biology of insulinomas. The second half of this review summarizes the clinical approach to the disorder. The diagnosis of insulinoma is established by demonstrating inappropriately high insulin levels with coincident hypoglycemia at the time of a supervised fast. Localization of insulinomas is challenging owing to their small size but should be attempted to maximize the chance for successful surgical resection and avoid risks associated with reoperation. In the majority of cases, successful surgical resection leads to lifelong cure.