Depression and diabetes are both common chronic disorders affecting people worldwide. Diabetes is characterised as a metabolic disorder often involving laborious self-management reduced health-related quality of life and often the prospect of diabetes related complications. Depression on the other hand, is a mood disorder associated with high levels of personal disability, lost quality of life, multiple morbidity and an increased risk of mortality. The adverse effects of depression may be especially detrimental in people with diabetes. This thesis presents the results of a programme of work beginning with two systematic reviews which demonstrate that the prevalence of depression is almost doubled in those with type 2 diabetes relative to those without and in addition is associated with worsened health related quality of life in those with comorbidity. The second phase of the research involved examining ethnic differences in the prevalence of depression as well as associations between depression and glycaemic control in secondary care patients with diabetes. Although depression did not explain ethnic differences in glycaemic control, the findings suggested that depression may be under-diagnosed in South Asians with diabetes. A need was highlighted to examine the prevalence of screen detected depression in a multiethnic population with type 2 diabetes relative to a suitable control group. The results from the final phase of the research concluded that although the prevalence of depression was not higher in primary care patients with type 2 diabetes compared to those without diabetes, the prevalence of depression in people with diabetes is nonetheless high, particularly in South Asians. Furthermore, findings from the final stage of the research conclude that depression is seriously under-diagnosed in people with type 2 diabetes, particularly in South Asians. The findings emphasise the need to improve the detection and management of depression to reduce inequalities in both depression and diabetes care.