Pancreatic islet isolation and transplantation are complicated procedures, indicated for a carefully selected group of patients. After isolation from the pancreas, the islets are infused into the portal vein. Allogeneic islet transplantation is performed in patients with diabetes mellitus, who suffer from severe hypoglycaemic events and/or progressive complications. One or more donor pancreases are used, which necessitates immunosuppressive treatment. In autologous islet transplantation, which is performed in patients in whom the pancreas has to be removed due to a non-malignant disease, the patients' own islets are isolated and reinfused. No immunosuppressive treatment is required. Reconstitution of endogenous insulin production in allogeneic islet transplantation leads to marked improvements in glycaemic regulation, protection against severe hypoglycaemic episodes and fewer diabetes-related complications. Autologous islet transplantation allows for preservation of endogenous insulin production, which prevents (unstable) diabetes from occurring. This article describes the indications, procedure and pitfalls of islet isolation and transplantation, including three representative cases.