Depression is a significant symptom for 25% of patients admitted to a palliative care unit, but many of these symptoms are not identified and therefore not treated. Reasons for failure to diagnose depression are misconceptions regarding low mood as being a normal part of a terminal illness and also the patients' reluctance to disclose their thoughts and feelings. Medical and nursing staff working within palliative care may also find difficulty in distinguishing between what could be called appropriate sadness and a treatable depressive illness. In an effort to improve the detection of depression, many professionals are using rating scales or tools in order to improve the diagnosis and treatment. This review discusses the complex issues of diagnosis of depression and highlights why certain tools may not be so useful or applicable in the palliative care population.