Beta‐cell regeneration represents a major goal of therapy for diabetes. Unravelling the origin of β cells during pancreatic regeneration could help restore a functional β‐cell mass in diabetes patients. This scientific question has represented a longstanding interest still intensively investigated today. This review focuses on pioneering observations and subsequent theories made 100 years ago and describes how technical innovation helped resolve some, but not all, of the controversies generated by these early investigators. At the end of the 19th century, complete pancreatectomy demonstrated the crucial physiological role of the pancreas and its link with diabetes. Pancreatic injury models, including pancreatectomy and ductal ligation, allowed investigators to describe islet function and to assess the regenerative capacity of the pancreas. Three main theories were proposed to explain the origins of newly formed islets: (i) transdifferentiation of acinar cells into islets, (ii) islet neogenesis, a process reminiscent of islet formation during embryonic development, and (iii) replication of preexisting islet cells. Despite considerable technical innovation in the last 50 years, the origin of new adult β cells remains highly controversial and the same three theories are still debated today.