Even 15 years ago, the accepted concept was that one was born with all the pancreatic ß cells one ever had. Many thought that insulin resistance would lead to diabetes without change in the ß cells. However, now the concept that diabetes only results when there is an inadequate functional mass of ß cells has gained general acceptance. The more obvious lack of ß cells is seen in type 1 diabetes as the result of autoimmune destruction of ß cells. The lack in type 2 diabetes with its hallmark of peripheral insulin resistance has been less obvious to many, yet the inability of the ß cells to match the increased demand for insulin can be seen as a lack of adequate functional mass. The evidence over the past decade has been compelling that in most cases the ß cells can, and do, compensate for added demand, resulting from pregnancy, obesity, or insulin resistance. It is important to remember that only 15–20% of people with obesity or severe insulin resistance become diabetic; the others maintain normoglycemia due to ß cell compensation. The new concept is that the ß cell mass is dynamic and increases and decreases both in function and mass to maintain the glycemic level within a very narrow physiological range. The changes in mass can be in both number (hyperplasia) and individual volume of ß cells (hypertrophy). When the mass cannot increase adequately, diabetes ensues. The question remains if such a compensation occurs during the several year long prediabetic phase of type 1 diabetes when ß cell autoantibodies are seen. The general acceptance of the concept of dynamic rather than staticßcell mass is based on clear evidence of changes in ß cell mass in both normal rodents and in “designer” mice (whether transgenic with overexpression or dominate-negative function or ablation of a gene), of varying replication rates of ß cells in vivo and in vitro, the presence of neogenesis or neoformation of islets postnatally and even in the adult, and physiological loss of ß cells indicating turnover. Further discussion of each of these areas follows.