Stress reactivity refers to a stable individual difference in response to stressors. This article addresses three questions about reactivity: (1) Are cardiovascular, endocrine, and immune responses to acute laboratory stressors stable over time and across stressor tasks? (2) Are cardiovascular, endocrine, and immune reactors the same people? and (3) Are reactive people more vulnerable to stressor-induced effects on susceptibility to infectious disease? We conclude that for many individual indicators of physiological responsiveness to stressors there is moderate stability over time and across stressor tasks indicating the possible existence of underlying dispositional characteristics; the commonality of immune and cardiovascular and hormonal responses to stress depend on the nature of regulation of the immune response being assessed; reactivity appears to have implications for vulnerability to stressor-associated disease risk (stress-by-reactivity interaction) in the natural environment, but the exact nature of this vulnerability is not as yet entirely clear.