Proteins do the work of cells. They provide structure, mediate signaling and catalyze nearly all the chemical reactions of physiology. A fundamental tenet of biochemistry is that protein functions derive from their unique structures. Moreover, changes in structure are thought to provide the basis for functional changes. We have used X-ray crystallography, an experimental technique that provides a snapshot of protein structures, to develop evidence that proteins populate large ensembles whose dynamic properties determine function. For the classic regulated enzyme ATCase, which initiates the synthesis of building blocks of DNA, the active form populates many distinguishable structures, while the inactive form populates a single unique conformation. These new data suggest that the catalytic cycle of ATCase requires flexibilty, and limiting molecular motions controls activity. This idea, called triggered release, contrasts with classic models of regulation based on switching between discreet, alternative, "on" and "off" structural states. In ongoing work, we are developing new approaches to define protein structural ensembles. By systematically sampling X-ray crystallographic maps of protein electron density, we are searching for small populations of specific, alternate protein conformations that previously have escaped detection. Such alternate native conformations may provide specific resevoirs of structural polymorphism that promote protein folding, function, signaling and evolution.