Two related ongoing research programs are described. The first examines optimal measurement strategies for assessing individual differences in cardiovascular reactivity, and the second investigates the effects of the social environment as a moderator of cardiovascular responding during psychological challenge. Models and evidence from related behavioral sciences (Psychometric Theory and Social Psychology, respectively) have provided useful guidance for this work. Relevant background and current data examining the measurement and determinants of cardiovascular reactivity are reviewed, with a focus on these multidisciplinary contributions. The implications of the work for the construct of reactivity and its possible role as a marker of disease risk are discussed.