The concept of pancreatic beta-cell mass is fundamental to the understanding of normal metabolism, the pathogenesis of diabetes, and the transplantation of beta-cell tissue. The amount of beta-cell tissue present in the pancreas is a major determinant of the quantity of insulin that can be secreted, and its mass will vary according to the size of the individual and the degree of insulin resistance present. Not all insulin-producing cells are the same, and the dimensions of this heterogeneity remain to be defined. Pancreatic beta-cell mass is markedly reduced in insulin-dependent diabetes mellitus and moderately reduced in non-insulin-dependent diabetes mellitus. In both forms of diabetes, there are qualitative and quantitative abnormalities of insulin secretion that cannot be explained entirely by changes in beta-cell mass. The amount of beta-cell tissue needed for successful transplantation has only been partially defined. Segmental (approximately 50% of the pancreas) transplantation can normalize plasma glucose levels in humans. Difficulty obtaining sufficient amounts of beta-cell tissue is expected to remain a barrier to successful islet transplantation for the immediate future. More should be learned about the function and fate of grafted islet cells.