Apoptosis of pancreatic beta cells is a feature of type 1 and type 2 diabetes, although by different effector mechanisms. In type 1 diabetes, beta cells are the targets of cytotoxic CD8(+) T cells that kill by releasing the contents of their cytotoxic granules into the immunological synapse with the target beta cell. In type 2 diabetes, the mechanisms of beta cell apoptosis are less clear, but believed to be due to cellular stresses including endoplasmic reticulum stress and oxidative stress induced by chronic exposure to high concentrations of glucose, lipids, inflammatory cytokines, or islet amyloid polypeptide. Measuring apoptosis in primary islets can be more difficult than in a beta cell line because islets exist as a cluster of cells and it is often difficult to obtain sufficient cells for any particular type of assay. Here, we describe two different methods for measuring islet cell apoptosis. The first method is the measurement of DNA fragmentation, a hallmark of apoptosis, of islets that have been cultured with reagents that induce stress. The second method is the measurement of islet lysis by activated cytotoxic T cells. We describe methods using mouse islets, but these can easily be adapted for human islets.