The Transtheoretical Model (TTM) is an integrative health behavior change theory that describes the process of how people change their behavior. The central organizing construct in the theory is stages of change, which are five distinct stages of readiness to change behavior, ranging from not ready to change (precontemplation), thinking about change (contemplation), preparing to change (preparation), changing (action), and maintaining the change (maintenance). Movement through the stages may be nonlinear, and cycling and recycling through the stages is viewed as a natural part of the change process. Other model constructs explain what drives individuals forward through the stages of change. Decisional balance involves a weighing of pros and cons of changing behavior, while self-efficacy involves situation-specific confidence that one can change. Increases in pros, deceases in cons, and increases in self-efficacy propel people forward through the stages of change. The processes of change are experiential and behavioral strategies that people use to change their behavior. In early stages of change, people use experiential strategies while they use behaviorally oriented strategies in later stages of change. The TTM holds significant implications for message design. Most notably, messages should be targeted and tailored to stages of change, and where possible, to other model variables as well. Studies indicate that the TTM has been successfully applied to health communication campaigns, and to a larger extent, to computer-tailored interventions to change health behavior. Meta-analyses indicate that scores of computer-tailored interventions have been efficacious, including many based upon the TTM and stages of change. New applications of the model include a focus on novel health behaviors, multiple behavior change, and advancing an understanding of message design in the context of the TTM in combination with other theoretical approaches.