Depression can appear before the onset of motor symptoms in Parkinson's Disease. Defects in the substantia nigra, basal ganglia and limbic structures are observed in the pre-clinical phase. Monoaminergic disfunction associates with emotional disorders in the early motor phase. We propose that the limbic loop of the basal ganglia and the lateral habenula play key roles in the early genesis of depression in PD. A better understanding of depression might facilitate the early diagnosis of Parkinson's disease.