Meta-analytic studies have established dispositional optimism as a personality trait with clear positive effects on psychological and physical well-being (Rasmussen, Scheier, & Greenhouse, 2009; Solberg Ness & Segerstrom, 2006). Optimism may be most important when experiencing stress, since optimists use more adaptive coping strategies and have more resources available compared to pessimists (Segerstrom, 2007; Solberg Ness & Segerstrom, 2006). Traditionally, studies have assumed optimism to be a stable trait, however recent studies have found optimism to be unstable in certain circumstances (e.g., Segerstrom, 2007). Although instability in optimism has been identified, research examining changes in optimism and well-being have been correlational and unable to identify causal associations. Additionally, these studies have not considered stress as a potential third variable to account for the associations between changes in optimism, and changes in well-being and health. The present study accounted for these limitations by replicating past findings, accounted for stress as a third variable, and also investigated the possibility that changes in optimism and stress may interact to influence changes in well-being and health. This was done by temporally investigating changes in optimism and stress at one point in time to predict changes in well-being and health at a later point in time. Additionally, this study investigated the extent to which the interaction between changes in optimism and stress at one point in time may influence the interaction between changes in optimism and stress on well-being and health at a later point in time. Results were able to show that changes in optimism did predict changes in well-being and heath after controlling for stress, but changes in optimism at one point in time did not predict changes in well-being or health at a later point in time after controlling for stress.