The present study focused on age and SES differences in stress reactivity in response to cognitively challenging tasks. Specifically, we assessed within-person trajectories of cortisol, a steroid hormone released by the adrenal gland in response to stressors, before, during, and after exposure to cognitively challenging tasks. We extend the current literature by simultaneously examining age and SES differences in physiological reactivity. Findings suggest that age and SES both play an important role in reactivity, such that it was the older adults with higher SES who were the most physiologically reactive to cognitive stressors. Implications of these findings for cognitive aging research are discussed. (Contains 1 figure and 3 tables.)