As the population ages, an increasing number of people are developing long-term conditions. This is a challenge for healthcare systems in terms of funding and support, and comorbidities add to the economic burden. This article considers the comorbid conditions diabetes and depression, which healthcare professionals increasingly encounter in clinical practice. It explores the reasons why diabetes and depression are associated, the influencing factors, and the bidirectional causes of diabetes and depression. In addition, the effects that a diagnosis of depression might have on a person having difficulty managing their diabetes are examined, and potential treatment strategies identified.