Models of personality and health have grown in complexity as more is discovered about how traits are related to health-related behaviors, morbidity, and mortality. The present article applies a model of personality and health that incorporates longitudinal relations, behavioral and physiological mechanisms, and context to the relation between Five-Factor Model personality traits and body mass index (BMI) and obesity (BMI≥30). Conscientiousness is associated consistently with lower body weight; the relation between the other traits and BMI is more complex. Conscientiousness is also associated with risk of obesity over time, and specific aspects of Conscientiousness and Neuroticism are associated with greater weight gain and are also sensitive to changes in weight over time. Behavioral (e.g., physical activity) and physiological (e.g., inflammation) factors explain part of the association between personality and BMI. Finally, the broader social environment shapes the expression of personality in relation to body weight. This review highlights replicable associations between personality and BMI and potential mechanisms of this association. Future research needs to better address how specific aspects of the social and family environment moderate the relation between personality and BMI and take a lifespan perspective to better incorporate how traits contribute to weight starting in childhood.