This chapter considers the concept of negative affect as it can be measured physiologically, and can serve as a unifying dimension across the anxiety disorder spectrum. It begins with an analysis of self-report data from nearly 300 anxiety patients and controls, considering three pertinent trait dimensions of psychopathology-fearfulness, anxiety, and depression-as they vary with principal diagnosis. Using a subsample of these participants, the relationship is assessed between status on these dimensions and a psychophysiological measure, potentiation of the probe startle reflex in the context of fearful imagery. The interview and questionnaire results suggest a hierarchy of disorders defined by increasing comorbid pathology and selfreport of negative affect. The psychophysiological findings show that, paradoxically, this ascending hierarchy of reported distress is directly associated with a reciprocal diminution in defensive reactivity to fear challenge.