There are roughly two philosophical literatures on “happiness,” each corresponding to a different sense of the term. One uses ‘happiness’ as a value term, roughly synonymous with well-being or flourishing. The other body of work uses the word as a purely descriptive psychological term, akin to ‘depression’ or ‘tranquility’. An important project in the philosophy of happiness is simply getting clear on what various writers are talking about: what are the important meanings of the term and how do they connect? While the “well-being” sense of happiness receives significant attention in the contemporary literature on well-being, the psychological notion is undergoing a revival as a major focus of philosophical inquiry, following on recent developments in the science of happiness. This entry focuses on the psychological sense of happiness (for the well-being notion, see the entry on well-being). The main accounts of happiness in this sense are hedonism, the life satisfaction theory, and the emotional state theory. Leaving verbal questions behind, we find that happiness in the psychological sense has always been an important concern of philosophers. Yet the significance of happiness for a good life has been hotly disputed in recent decades. Further questions of contemporary interest concern the relation between the philosophy and science of happiness, as well as the role of happiness in social and political decision-making.