People with intellectual disability (ID) were once considered immune to developing psychopathology, including affective disorders such as depression. Now research has shown that people with ID do suffer from depression, and the focus is on understanding how to best diagnose and provide treatment. Research has come a long way in adapting the diagnostic process for people with mild/moderate deficits in intellectual functioning and communication abilities. However, there is a lack of research and clinical understanding regarding how to assess depression in people who lack communication skills and function with severe or profound deficits in intellectual functioning. This paper reviews the available literature on the occurrence of depression in people with ID, specifically those with more significant deficits, and discusses the difficulties that arise in trying to determine a diagnosis of depression for this population.