Considerable theory and research on positive affect (PA) reveals that high PA relates to adaptive outcomes. Increasingly, however, it has become clear that high PA also has a costly side, as it sometimes relates to adverse outcomes, such as intense psychological distress, risky health behaviors, and even early mortality. This chapter appraises the existing body of empirical evidence and discusses how frameworks that consider both stable and dynamic forms of PA can help reconcile conflicting evidence concerning the association between PA and diverse health outcomes. Drawing on survey, daily diary, and biological data from the Midlife in the United States (MIDUS) study, empirical findings and ongoing studies of middle-aged adults are summarized. The chapter concludes with a discussion of integrative research opportunities afforded by MIDUS.