After a few pairings of a threatening stimulus with a formerly neutral cue, animals and humans will experience a state of conditioned fear when only the cue is present. Conditioned fear provides a critical survival-related function in the face of threat by activating a range of protective behaviors. The present review summarizes and compares the results of different laboratories investigating the neuroanatomical and neurochemical basis of conditioned fear, focusing primarily on the behavioral models of freezing and fear-potentiated startle in rats. On the basis of these studies, we describe the pathways mediating and modulating fear. We identify several key unanswered questions and discuss possible implications for the understanding of human anxiety disorders.