Health promotion commonly involves altering health behaviors, but changing behavior proves challenging and often frustrating for clinicians involved in health promotion, whether at the individual or community level. This chapter summarizes a variety of conceptual approaches to understanding the stickiness of health behaviors, reviewing how behaviors are maintained by numerous, interacting factors. In place of a traditional linear, additive model of the influences on health behavior and behavior change, the chapter highlights a systems perspective on the interactions among multiple influences. More specifically, it argues that these influences can be viewed as a Complex System of Systems, or CASoS. To illustrate these ideas, dietary change for weight loss is used as an example, and the process of changing diet is described through a modified Transtheoretical Model of the stages of change.