The liberation of calcium ions from the endoplasmic reticulum into the cytosol through inositol trisphosphate receptor/channels (IP3R) plays a central role in regulating numerous forms of cellular activity.  Recent advances in imaging technology reveal that cellular calcium signals are constructed from elementary release events (“puffs”), involving calcium flux through single channels or clusters of small numbers of channels.  The activity of individual channels within a cluster is coordinated by Ca2+ diffusion and calcium-induced calcium release (CICR).  Cluster-cluster interactions by successive cycles of calcium diffusion and CICR may give rise to global calcium waves.  The hierarchy of spatial and temporal scales relevant to these processes spans more than 6 decades (microseconds to minutes; nano-meters to millimeters).  It is impossible to resolve all these scales simultaneously in a single experiment.  Moreover, the shorter time and distance scales cannot be resolved by any available experimental approaches.  We therefore propose to use a dual, tightly integrated approach of mathematical modeling and high-resolution cellular calcium imaging to illuminate the mechanisms by which the elementary events are triggered and coupled to produce cellular calcium signals.  This will be done for a single, well characterized model cell system (the Xenopus oocyte), but the emergent principles will be widely applicable to IP3 signaling across many cell types and species, as well as to calcium signaling mediated by ryanodine receptors.  By combining data obtained from patch-clamp, superfusion and imaging experiments we aim to develop a comprehensive model that is consistent with a multitude of observations, has predictive value, and extends to crucial – but experimentally inaccessible – space and time scales.