A motivational system approach to avoidance behavior is presented. According to this approach, a motivational state increases the probability of relevant response patterns and establishes the appropriate or “ideal” consummatory stimuli as positive reinforcers. In the case of feeding motivation, for example, hungry rats are likely to explore and gnaw, and to learn to persist in activities correlated with the reception of consummatory stimuli produced by ingestion of palatable substances. In the case of defense motivation, fearful rats are likely to (...) flee or freeze, and to learn to persist in activities correlated with consummatory stimuli produced by flight from a dangerous place. Defense system activation is distinct from alarm reactions. The latter prepare the animal for probable noxious events, involve relatively intense negative affect and extinguish rapidly in situations where the noxious event no longer occurs. In contrast, defense system activation potentiates innate and modified defense reactions, thus preparing the animal for possible, but not necessarily probable, noxious events; it involves little or no negative affect and extinguishes very slowly when the noxious event no longer occurs. With these assumptions and the resulting model we attempt to resolve several long-standing problems in avoidance learning, including the low correlation between negative affect and avoidance performance, differential rates of extinction for avoidance performance and conditioned emotional responses, and evidence that some avoidance responses are much more easily learned than others. In addition, the model has implications for the study of parallels between appetitive and aversive motivation, sign tracking in aversive conditioning, and orientation of flight responses. Historical antecedents and alternative approaches are discussed. (shrink)