Abstract of the PhD thesis: Virtual worlds such as Second Life and World of Warcraft are increasingly playing an important role in the lives of many people, currently counting more than 40 million active people who communicate, do business, fall in love, express themselves, build homes and generally seek experiences that are difficult if not impossible to have in the real world. This dissertation takes up the question of when and how such activities can affect the quality of our lives. In addressing this question, Søraker partly employs philosophical analyses of the presuppositions inherent in our conceptions of well-being, of the differences and similarities between virtual worlds and actual reality, as well as general clarifications of what virtual worlds are and what kinds of experiences and opportunities they offer. He also proposes a new theory of well-being, entitled Confidence Adjusted Intrinsic Attitudinal Hedonism, which is better equipped to make sense of the peculiar epistemological, ontological, ethical and prudential nature of virtual worlds. In addition to this, he also draws heavily on psychological research on subjective well-being, and critically examines what these results imply with regard to the value of virtual worlds in such areas as sensory pleasure, love and friendship, engaging in skill-demanding activities and having a sense of meaningfulness. The ultimate purpose of the dissertation is to leave the reader with a more comprehensive and nuanced basis for understanding the role that virtual worlds and entities can and ought to have in a good life.