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The study of relationships between personality traits and health has a long history in psychosomatic research. However, personality science has evolved from an understanding of personality as fixed traits to one that acknowledges that personality is dynamic. Dynamic approaches to conceptualizing and measuring personality and individual differences can enrich personality-health research. In this Presidential Address (American Psychosomatic Society, 2018), I consider how different formulations of personality—stable traits, stable signals in a noisy or variable measure, within-person changes, and intraindividual variability—can be implemented to better understand how personality is related to health and particularly to immune function. These approaches recognize and, in some cases, capitalize on the fact that personality factors can display variability as well as stability over time. They also require repeated measurement and therefore greater methodological sophistication that considers reliability and generalizability, Simpson's paradox, and the difference between variability and flexibility. Dynamic qualities of personality and individual differences potentially influence health, and designs and methodology that incorporate them can illuminate the important processes that occur inside the error bars.