In this chapter, I will review several theories whose purpose is to integrate and unify ideas in QOL research. These include livability theory, capability theory, stocks and flows, the joyless economy, quality of the person + environment, homeostasis, QOL = happiness + life satisfaction + absence of ill-being, the bidirectional spillover model, dynamic well-being, ontological well-being and the 3P model, and the psychology of quality of life.