Pancreatic δ cells act locally to repress both insulin and glucagon secretion. Because they are a rare cell type, experimentation examining δ-cell function and control has lagged that of the more abundant α and β cells. Emerging evidence, enabled partly by developing single-cell technology, demonstrates that δ-cell function is, in part, directed by δ cells but that δ cells also have intrinsic control. The contribution of these cells to overall glucose homeostasis and diabetes onset and progression is still unclear. However, they regulate both α and β cells, both of which are dysfunctional in diabetes, and their numbers are disrupted in humans with diabetes and in multiple animal models of diabetes, suggesting δ cells are a pivotal character in both health and disease.