The philosophical template of eudaimonia is of ancient Greek heritage and has to do with completeness, fulfillment, and excellence; a non-reductionistic conceptualization of the perfect life that can be summarized as “a complete state of being and doing well” (Sumner, Welfare, happiness, and ethics. Clarendon Press, 1996, p. 69). Many theories of philosophical eudaimonism exist, but the Aristotelian version is the most celebrated. He defined eudaimonia normatively, as rational activity in accordance with virtue. This idea has been described as “man-as-he-could-be-if-he-realized-his-essential-nature” (MacIntyre, After virtue, University of Notre Dame Press, 2007, p. 52). The Aristotelian notion of human nature is not easily translated to modern science, but the concept of organismic living might be a good place to start. An important element in organismic thinking is that all living organisms have an inborn tendency to activate and regulate their behavior toward some goal—to actualize their nature as autonomous, self-organizing systems. Organismic living is also highly interactionist, with sophisticated collaborations occurring within and between units. Humanistic and clinical psychologists in the mid-twentieth century were inspired by the notion of organismic living and used the concept to develop theories of self-actualization and fully functioning individuals. These theories were not explicitly described as eudaimonic, but they stimulated what we may label the first generation of eudaimonic wellbeing (EWB) theories in psychology. The most influential among the early EWB theories were developed by Waterman, Ryff, and Deci and Ryan. Waterman’s theory of personal expressiveness is inspired by humanistic psychology and the philosopher David Norton and his ethical individualism. Norton’s controversial interpretation of eudaimonism suggests that we all have a moral responsibility to discover “the daimon” within ourselves and to live in accordance with it. Waterman also follows Norton in identifying eudaimonia as the feeling that accompanies efforts of self-realization and the development of one’s true potential. These feelings are referred to by Waterman as expressive and are contrasted with hedonic enjoyment. The idea that eudaimonia can be defined as a feeling is questioned. Ryff’s psychological wellbeing theory (PWB) comprises six dimensions: autonomy, environmental mastery, personal growth, positive relations, purpose in life, and self-acceptance. The theory is often taken to be eudaimonic, but only personal growth and purpose in life are explicitly identified as carriers of eudaimonic meaning. The chapter criticizes Ryff’s theory for being underspecified and for downplaying the importance of affective and evaluative indicators of wellbeing. Deci and Ryan’s self-determination theory (SDT) suggests that the fulfillment of our basic psychological needs for autonomy, competence, and relations will lead to increased wellbeing. According to SDT, eudaimonia is intimately linked with the concept of being fully functioning. However, the concept of full functioning appears idealized and difficult to study empirically. Moreover, it is unclear whether SDT considers psychological need fulfillment to be a kind of wellbeing or a predictor of wellbeing. Theories of EWB, other than those of Waterman, Ryff, and Deci and Ryan, offer a plethora of eudaimonic dimensions and indicators, but two broad and overarching dimensions have been identified: (1) growth/self-realization/self-actualization/development of potentials/full functioning/maturity and (2) meaning/purpose/long-term perspective/caring. The concept of personal growth is historically rooted in the idea Germans refer to as “Bildung,” a personal transformation enabled through certain experiences that includes the formation of intellectual, personal, and moral capacities. Meaning or purpose in life cannot be directly related to Aristotelian eudaimonism but is relevant for EWB as a means of fulfilling one’s values. For example, Viktor Frankl’s notion of a “will to meaning” is a kind of self-transcendence that seems compatible with the eudaimonic idea of fulfillment. Many theories of EWB have been proposed, but little conceptual agreement has been reached. One reason is that concepts typically involved in eudaimonic models are difficult to test empirically. Several popular approaches are grounded in an idealized, a priori conceptualization of eudaimonia on the one hand and a set of self-report scales with unclear relations to the concepts on the other. Thus, the empirical part of eudaimonic research does not speak well with the theoretical part. The combination of rampant fauna of untestable concepts and a cottage industry of theory-drained scale production makes it unlikely that a conceptual consensus about eudaimonic wellbeing will be reached anytime soon.