Parkinson's disease (PD) is a neuropsychiatric disease affecting approximately 1-2% of the general population. The classical triad of symptoms, tremor, rigidity, and bradykinesia is mainly caused by degeneration of dopaminergic neurons from the substantia nigra. However, other neurotransmitter systems also show signs of degeneration, among which the serotonergic system. The exact role of serotonin in PD remains unclear. We present here a review about functional serotonergic interventions and serotonergic imaging studies in PD, and will go into the importance of combining preclinical and clinical research data in order to gain more insight into the role of serotonin in PD. More specifically, the present review is aimed at bridging the gap between data from animal models of PD and data from human research.