Health behaviour theories are central to understanding health behaviour change because they identify key determinants of health behaviour and the processes by which the determinants influence behaviour change. Theoretical models are important for designing and evaluating health behaviour change interventions because they provide guidance on which variables to intervene on (key determinants) and a framework to understand why an intervention does (or does not) work (processes). In this entry, eight different theoretical models of health behaviour change (theory of planned behaviour, health belief model, social cognitive theory, the COM-B model, self-determination theory, the health action process approach, the transtheoretical model, and the precaution adoption process model) are described and evidence for interventions based on the models is summarised.