According to set point theories of happiness and well-being, each individual possesses a person-specific equilibrium level of well-being that is relatively stable throughout life. It is proposed that even after experiencing major life events (be these positive or negative in nature), individuals return [Page 1958]to their set point. Changes in life circumstance may thus have short-term but not long-lasting and sustainable effects. In this entry, we use set point theories as an umbrella term for theories of happiness and well-being that share these postulations, including adaptation level, hedonic treadmill, set point, and dynamic equilibrium theories. This entry describes assumptions of set point theories, summarizes previous research on stability and continuity as well as instability and discontinuity of happiness and well-being across life, and addresses implications ...