This chapter provided an overview on models that explain change processes in health behavior and change toward more physical activity. It is divided into four main parts. In the first section, models are introduced that focus on the motivational process of behavior change and how individuals’ decision-making is influenced. The second section deals with theories of action execution—the part of health behavior change in which a person implements intentions into action. These models also focus on action and coping planning to overcome obstacles for intention implementation. In the third section, we address stage and process models of health behavior change that unify processes of motivation and action execution in holistic models. Finally, in the fourth section, dual-process theories are introduced that assume that human health behavior change is affected by at least two types of information processes that are automatic and reflective, as well as integrated models of health behavior, that integrate constructs from multiple theories.