There is little longitudinal information on aging-related changes in emotional responses to negative events. In the present article, we examined intraindividual change and variability in the within-person coupling of daily stress and negative affect using data from 2 measurement-burst daily diary studies. Three main findings emerged. First, average reactivity to daily stress increased longitudinally, and this increase was evident across most of the adult lifespan. Second, individual differences in emotional reactivity to daily stress exhibited long-term temporal stability, but this stability was greatest in midlife and decreased in old age. Third, reactivity to daily stress varied reliably within-persons (across-time), with individuals exhibiting higher levels of reactivity during times when reporting high levels of global subject stress in the previous month. Taken together, the present results emphasize the importance of modeling dynamic psychosocial and aging processes that operate across different time scales for understanding age-related changes in daily stress processes.