Research suggests that co-morbid diabetes and depression is common; however, the implications for clinical practice remain unclear. This paper reviews the current epidemiological evidence on comorbid diabetes and depression, in order to identify the key publications which could both inform practice and identify gaps in knowledge and research. A systematic review was conducted to identify published literature on the epidemiology of diabetes and depression. In order to review evidence on up-to-date knowledge of recent research and innovations in care literature searches for the last five years (August 2006-August 2011) were conducted. To identify relevant literature, electronic databases MEDLINE, Psych-INFO and EMBASE were searched for English language articles in peer-reviewed journals. High rates of co-morbidity of depression and diabetes have been reported. The prevalence rate of depression is more than three-times higher in people with type 1 diabetes (12%, range 5.8-43.3% vs. 3.2%, range 2.7-11.4%) and nearly twice as high in people with type 2 diabetes (19.1%, range 6.5-33% vs. 10.7%, range 3.8-19.4%) compared to those without. Women with diabetes and also women without diabetes experience a higher prevalence of depression than men. Reviewed studies provide support for a modest relationship between diabetes and depressive symptoms, but the exact direction of this relationship remains unclear. Most studies reviewed were cross-sectional and this limits any conclusions about the causal nature and direction of the relationship between diabetes and depression. Variation in measurement methods, lack of longitudinal data and few studies outside Europe and America limit the generalizability of the findings of this review. Current research suggests that the risk of developing depression is increased in people with diabetes; however, further studies are required in order to establish the nature of the relationship between depression, glycaemic control and the development of diabetes complications, and make appropriate recommendations for treatment and to support self-management of diabetes. Copyright © 2012 Elsevier B.V. All rights reserved.