Abstract This article discusses the overgeneralized construct of ‘arousal’, pointing out some of the problems of speaking of it as mediated by a single system. The literature and current studies of the orienting and defensive reflexes, and augmenting-reducing of the cortical evoked potential in relation to the sensation-seeking (SS) trait are discussed. The data relating platelet MAO and CSF, plasma and urinary measures of bioamines, their metabolites and enzymes, to SS, extraversion and behavioral indices are also treated. Relationships between arousal measured in different systems are pointed out. Personality theory is seen as a vehicle for understanding the organization of arousal systems in adaptation to environmental stimulation.