Context: Insulin resistance can be compensated by increased functional pancreatic β-cell mass; otherwise, diabetes ensues. Such compensation depends not only on environmental and genetic factors but also on the baseline β-cell mass from which the expansion originates. Objective: Little is known about assembly of a baseline β-cell mass in humans. Here, we examined formation of β-cell populations relative to other pancreatic islet cell types and associated neurons throughout the normal human lifespan. Design and Methods: Human pancreatic sections derived from normal cadavers aged 24 wk premature to 72 yr were examined by immunofluorescence. Insulin, glucagon, and somatostatin were used as markers for β-, α-, and δ-cells, respectively. Cytokeratin-19 marked ductal cells, Ki67 cell proliferation, and Tuj1 (neuronal class III β-tubulin) marked neurons. Results: Most β-cell neogenesis was observed preterm with a burst of β-cell proliferation peaking within the first 2 yr of life. Thereafter, little indication of β-cell growth was observed. Postnatal proliferation of α- and δ-cells was rarely seen, but a wave of ductal cell proliferation was found mostly associated with exocrine cell expansion. The β-cell to α-cell ratio doubled neonatally, reflecting increased growth of β-cells, but during childhood, there was a 7-fold change in the β-cell to δ-cell ratio, reflecting an additional loss of δ-cells. A close association of neurons to pancreatic islets was noted developmentally and retained throughout adulthood. Negligible neuronal association to exocrine pancreas was observed. Conclusion: Human baseline β-cell population and appropriate association with other islet cell types is established before 5 yr of age.