Functional islet cell tumors cause recognizable clinical syndromes based on the peptide products they secrete (Table 1). Frequently, the diagnosis of these tumors is delayed. A high index of suspicion coupled with the use of appropriate biochemical and provocative tests should lead to earlier diagnosis. A suggested management plan for patients suspected of having an islet cell tumor is shown in Fig. 1. Earlier diagnosis coupled with advances in preoperative and intraoperative localization techniques have resulted in an increased number of patients being cured.