Non-motor symptoms, such as psychiatric symptoms and autonomic dysfunction, are common co-morbid conditions in Parkinson's disease (PD) and major contributors to poor quality of life and disability. Within the group of neuropsychiatric conditions, depressive symptoms are the most common condition. Despite their frequency and importance, depressive symptoms can be difficult to assess and diagnose and thus depression in PD is frequently unrealized. Diagnostic challenges include the overlap of depressive symptoms with motor and non-motor symptoms of PD, such as dementia and apathy. Furthermore, there are no definite standards to assess and diagnose depression in PD leading also to the lack of exact data on the epidemiology of this non-motor symptom in PD. Depending on the diagnostic test and the study design the prevalence of depression in PD is reported between 7 and 72% of PD patients with approximately 40% in most cross-sectional studies. In contrast, the pathogenesis and long-term course of depression in PD remain elusive. Current hypothesis, however, includes that depressive symptoms are part of the core condition of PD when regarded as an entity. The present review summarizes the current knowledge on epidemiology, pathogenesis and diagnosis of depression in PD and proposes on this data base a standard procedure for screening and diagnosis of depressive symptoms in PD.