This chapter reviews philosophical foundations of the eudaimonic approach, and along the way notes its distinctiveness from hedonic conceptions of psychological well-being. It talks about the psychological foundations of the eudaimonic approach, which were built on the integration of multiple conceptions of positive functioning from clinical, development, existential, and humanistic psychology. The chapter highlights growing evidence of the protective value of eudaimonic well-being for physical health, and then turns a reader's attention to the relevance of eudaimonia for mental health. It summarizes findings that have linked traditional indicators of mental illness to eudaimonic well-being. The chapter also discusses evidence of interventions to promote eudaimonic, both in clinical contexts and in educational contexts. A main message is that efforts to facilitate experiences of purposeful engagement, self-realization, and growth are vital avenues for improving human lives.