Without doubt happiness is the central concept on which ancient moral philosophy was found. Christian authors' approach to philosophy is very much shaped by their understanding of happiness. The author first sketches out the basic characteristics of Aristotelian happiness. Afterwards, he briefly examines Albert the Great's commentaries on the EN. Aquinas's interpretation of Aristotelian happiness in his own commentary, the Sententia Libri Ethicorum(SLE), can be understood at least partly as a critical reaction to the highly influential reading of his teacher. The author outlines Aquinas's understanding of happiness, starting from his commentary and proceeding to the theological works. This enable to finally evaluate the way in which Aquinas's theological background shaped his reading of Aristotelian eudaimonia. Aristotle generally sticks to the idea that virtuous activity is the essential and constitutive part of happiness.