In this reappraisal of the work of Pierre Janet at the centenary of the publication of L'automatisme psychologique, the authors review his investigations into the mental processes that transform traumatic experiences into psychopathology. Janet was the first to systematically study dissociation as the crucial psychological process with which the organism reacts to overwhelming experiences and show that traumatic memories may be expressed as sensory perceptions, affect states, and behavioral reenactments. Janet provided a broad framework that unifies into a larger perspective the various approaches to psychological functioning which have developed along independent lines in this century. Today his integrated approach may help clarify the interrelationships among such diverse topics as memory processes, state-dependent learning, dissociative reactions, and posttraumatic psychopathology.