Secretion, whether from nerve terminals or hormone-secreting cells, is determined by the product of the number of release-ready vesicles and the probability of release per vesicle. The probability of release is in turn dependent on both the concentration of calcium seen by the vesicles and the affinity of the release mechanism for calcium. All of these factors vary in time, depend on conditions and history of stimulation, and vary among cell types. Vesicle trafficking to releases sites on the plasma membrane is regulated by calcium and also by metabolism, at least in insulin-secreting cells. It has long been known that vesicles differ in proximity to calcium channels, but recent evidence from many cell types supports the hypothesis that a subset of vesicles distant from channels may have enhanced sensitivity to calcium and play a larger role than previously thought. These issues will be discussed based on models in beta cells along with possible relevance for pituitary and hypothalamic neurons.