Over the past 12 years, the US Food and Drug Administration (FDA) has reviewed more than 40 investigational new drug applications for the use of allogeneic pancreatic islets to treat type 1 diabetes mellitus. Recent advances in islet cell isolation, transplantation, and immunosuppressive maintenance have led to multiple centers reporting promising results in the treatment of type 1 diabetes with allogeneic islet cells. The FDA held an advisory committee meeting on October 9-10, 2003, to explore the potential for licensing allogeneic islets as a therapy for severe type 1 diabetes. This article highlights the manufacturing challenges, discussed by the FDA advisory committee, that remain to be resolved before allogeneic islets can be approved for treatment of type 1 diabetes. This article also briefly addresses the challenges facing the clinical trial design of studies that could be used to support licensure.