The presence or absence of a particular experience at a particular time in the life cycle may exert an extraordinary and dramatic influence over structure or function well beyond that point in development. Such sensitive periods are thought to be widespread in animal and in human neurobiology and psychology. A comprehensive treatment of the sensitive period needs minimally to include information about its structural characteristics as well as an interpretation of its causes, including why the sensitive period arises in terms of the natural history of the species and how the sensitive period is regulated in terms of physical, physiological, and psychological processes. This article provides a framework for research and theory concerning specific sensitive periods and the sensitive period generally conceived. The framework delimits four sets of parameters, which encompass 14 structural characteristics that define sensitive periods, and two levels of causal interpretation that guide research and theory into sensitive periods however they may be manifested.