Compared to anxiety disorders, posttraumatic stress disorder (PTSD) may be more clearly attributable to discrete, aversive learning events capable of evoking both conditioned fear responding to stimuli associated with the event and more general overreactivity—or failure to adapt—to intense, novel, or fear-related stimuli. The relatively straightforward link between PTSD and these basic, evolutionarily old learning processes of conditioning, sensitization, and habituation affords models of PTSD comprising fundamental, experimentally tractable mechanisms of learning that have been well characterized across a variety of mammalian species including humans. Though such learning mechanisms have featured prominently in explanatory models of psychological maladjustment to trauma for at least 100 years, much of the empirical testing of these models has occurred only in the past three decades. This chapter delineates the variety of theories forming this long-standing tradition of learning-based models of PTSD; details empirical evidence for such models; attempts an integrative account of results from this literature; and delineates limitations of, and future directions for, studies testing learning correlates of PTSD.