The transtheoretical model of behavior change has become one of the most influential theoretical models within health psychology. Characterized by treating behavior change as a dynamic process, it has recently been applied to diabetes mellitus. Both interventional and observational studies have applied the transtheoretical model to the care of patients with diabetes in multiple settings; the studies that have been performed have been very heterogeneous in nature. Pooled data from these studies yielded a distribution that describes patients across the stages of change, the main construct of the transtheoretical model. Regarding treatment effectiveness, the empirical data reported in the studies showed that participants did progress through the stages of change when a transtheoretical model-based intervention was applied, and that a higher percentage of participants progressed in the intervention groups than in the control groups. Further research is needed in this field to obtain more information about the constructs of the transtheoretical model and the stages of change. The use of validated questionnaires in the assessment process is also recommended. The studies included in this review show how the transtheoretical model has made a positive contribution to interventions in the field of diabetes.