This entry outlines various components of diabetes mellitus, including etiology, health complications, risk factors, preventative measures, associated psychosocial factors, and issues related to the management and treatment of diabetes. Specifically, we provide a definition of both type 1 and type 2 diabetes, including symptomology, health consequences, prevalence rates, and demographics of vulnerable populations. A description of less common types of diabetes, including gestational, cystic fibrosis-related, and monogenic diabetes, is also incorporated. We review both behavioural and psychological risk factors of diabetes, including smoking, sedentary behaviours, and the role of depression and anxiety. Additional risk factors for diabetes that may cluster with poor health behaviours are described, including excess weight and poor dietary habits. Furthermore, we delineate effective evidenced-based prevention strategies that have been evaluated and that emphasise both behavioural lifestyle interventions, as well as pharmacological treatments. Specifically, previous studies indicate that engaging in physical activity and eating a healthy diet are protective against diabetes. We address the role of anxiety, eating disorders, and socioeconomic status as important psychosocial factors that have been associated with poor management of diabetes. Differences in diabetes complications among ethnic/minority populations are presented. We also delineate the role of family and friends and the importance of providing social support and autonomous support in the treatment and management of diabetes. Multiple approaches for improving treatment adherence are discussed, including the importance of self-regulation and self-monitoring. Interventions are summarised that highlight the importance of behavioural lifestyle interventions and pharmacological interventions for managing and preventing the progression of diabetes.