This chapter shows another theory of well-being: the happiness theory of well-being (HTWB). The term 'happiness' is deployed in a wide variety of ways across ancient philosophy, psychology, welfare economics and contemporary philosophy. The exercise and development of the capacities essential to humans conditioned on their being living things is good for humans. The diminishment of the capacities essential to humans conditioned on their being living things is bad for humans. The second claim plausibly attributes a disposition or tendency towards the state of being happy. The relation between the first and second ways of using 'happy' is not problematic. The first attributes a certain kind of state to a person. The second attributes a disposition of the person to being in that state. According to HTWB and PTH, if one is at a certain hedonic level then one is necessarily at a certain happiness level and level of well-being.