In computer science, the concept of user experience has proven to be beneficial in order to improve the quality of interaction between software and its users, by taking users’ emotions and attitudes into account. In general, user experience focuses on interaction. As not only interaction (e.g., good usability) is of importance for players, this chapter discusses how the concept of user experience can not only be applied to serious games, but also how it can be extended in order to cover the characteristics of games as a special software. For this refined concept, the term player experience has been coined. First, the concept of player experience is introduced in this chapter. The adequate conceptualization of player experience requires differentiating specific dimensions like (game-) flow, immersion, challenge, tension, competence, and emotions. Because of the individual nature of player experience, psychological models need to be used for the conceptualization as they are able to reflect this multidimensional structure. In addition, interdisciplinary models are needed in order to address the various factors influencing player experience. This ensures a holistic approach. Second, the question how to measure player experience is discussed. Here, different levels have to be distinguished: Behavior, physiological reactions, and subjective experience. Finally, it is shown how knowledge about player experience can be employed to develop serious games systematically and to improve their quality.