The study of well-being is hampered by the multiplicity of approaches, but focusing on a single approach begs the question of what "well-being" really is. We analyze how well-being is defined according to the three main kinds of theories: "Liking" approaches (generally adopted by psychologists), "Wanting" approaches (predominant among economists), and "Needing" approaches (used in both public policy and psychology). We propose an integrative framework, the engine model of well-being, drawing on Seligman (Seligman, M. E. P., 2011. Flourish. New York, NY: The Free Press) and Sen's (Sen, A. K., 1999, Development as freedom. Oxford, England: Oxford University Press) emphasis on the plurality of this construct by distinguishing among (a) inputs (resources that enable well-being), (b) processes (internal states of mechanisms influencing well-being), and (c) outcomes (the intrinsically valuable behaviors that reflect the attainment of well-being). We discuss implications for research, measurement, and interventions.