Well-being refers to a person’s hedonic experience of feeling good and to the eudaimonic experience of fulfillment and purpose. Employee well-being is influenced by experiences at work and, in turn, has an effect on behavior at work such as task performance and other on-the-job behaviors. In this article, I describe well-being as a dynamic construct that changes over time and fluctuates within a person. I review and integrate longitudinal, experience-sampling, and related research on well-being change and variability. I address the role of job stressors, job resources, the interpersonal environment, personal resources, the work–home interface, and performance. I discuss questions of affect symmetry, homology of the between-person and within-person level, and reciprocity between well-being and other variables. The article concludes with suggestions for future research.