A nxiety sensitivity (A S) is the fear of anxiety-related sensations. It is thought to arise from beliefs that these sensations have harmful co n sequences (Reiss 1991; Reiss & M cNally, 1985). This description of A S - as a pathological way of v iew ing and reacting to anxiety sym ptom s-certainly sounds like som ething that must have been learned. However, AS theorists have been careful not to imply that this is the case. In fact, the literature on A S is profoundly bereft of studies of the origins of A S - studies that ask the question, “H ow does som eone develop high A S ?" This leaves open the possibility that A S might be a biological characteristic, much like height or intelligence, which may be predominantly inherited rather than learned. This chapter does not directly address this possibility mainly because definitive data are lacking. What it does attempt to do is review studies that sought to exam ine biological aspects of anxiety sensitivity in the hope that this can inform the discussion and lead to a testable series of questions about the nature of A S.