Personalization and adaptivity can promote motivated usage, increased user acceptance, and user identification in serious games. This applies to heterogeneous user groups in particular, since they can benefit from customized experiences that respond to the individual traits of the players. In the context of games, adaptivity describes the automatic adaptation of game elements, i.e., of content, user interfaces, game mechanics, game difficulty, etc., to customize or personalize the interactive experience. Adaptation processes follow an adaptive cycle, changing a deployed system to the needs of its users. They can work with various techniques ranging from simple threshold-based parameter adjustment heuristics to complex evolving user models that are continuously updated over time. This chapter provides readers with an understanding of the motivation behind using adaptive techniques in serious games and presents the core challenges around designing and implementing such systems. Examples of how adaptability and adaptivity may be put into practice in specific application scenarios, such as motion-based games for health, or personalized learning games, are presented to illustrate approaches to the aforementioned challenges. We close with a discussion of the major open questions and avenues for future work.