Health psychology is a relatively recent field which evolved in the latter part of the twentieth century. It has become a central element in attempts within public health to promote health, modify negative health behaviors, influence behavior change, aid in the prevention of ill health and disease, and assist in the maintenance of good health. The epidemiological transition has described a shift in causes of mortality from communicable to non-communicable diseases which highlights the importance of behavior change in managing modifiable lifestyle factors which strongly influence chronic disease. Many models and theories have been developed which attempt to explain why people behave in certain ways, how they may change behavior, what internal and external factors may influence their actions, and how they may develop and utilize their own cognitions of health and disease when faced with ill health. There are too many models and theories to cover in one chapter, so the focus here is on some of the most widely used, established, and thoroughly researched models. The Transtheoretical Model, the Health Belief Model, the Theory of Planned Behavior, the Behavior Change Wheel with COM-B system, and the Common-Sense Model of Self-Regulation are discussed. The brief history of their development and evolution and examples of how they have been applied in certain areas of health are given to demonstrate their practical use in explaining health behaviors and in influencing behavior change. Acknowledgments of their limitations and criticisms are also given to provide a balanced overview of each featured model.