The ability to predict the likelihood of an aversive event is an important adaptive capacity. Certainty and uncertainty regarding pain cause different adaptive behavior, emotional states, attentional focus, and perceptual changes. Recent functional neuroimaging studies indicate that certain and uncertain expectation are mediated by different neural pathways-the former having been associated with activity in the rostral anterior cingulate cortex and posterior cerebellum, the latter with activation changes in the ventromedial prefrontal cortex, mid-cingulate cortex and hippocampus. Expectation plays an important role not only in its modulation of acute and chronic pain, but also in other disorders which are characterized by certain expectation (specific phobias) or uncertain expectation (generalized anxiety disorder) of aversive events.