Due to the shortcomings in understanding humor, a state-trait model of cheerfulness, seriousness and bad mood was introduced to describe the temperamental basis of the sense of humor [1-4]. This chapter sketches the development and characteristics of the postulated state-trait model and presents its relationship to different models of the sense of humor. Literature will be reviewed that shows that trait cheerfulness accounts for most variation in existing self-report assessment tools of the sense of humor. Further, the relation of trait cheerfulness to health and well-being related variables (e.g., flourishing ; coping  and life satisfaction, ) will be discussed. Attention is given to experimental and correlational evidence, which shows that trait cheerfulness is positively related to adaptive coping mechanisms, positive experience and well-being. This is particularly interesting for cheerfulness interventions to fostering well-being and overcoming adversities. Finally, implications for the study of positive traits and respective interventions will be discussed.