The identification of several monogenic forms has established Parkinson disease (PD) as a movement disorder with a considerable genetic origin in at least a subset of patients. Four of the known forms, Parkin-, PINK1 (PTEN-induced putative kinase 1)-, DJ1-, and LRRK2 (leucine-rich repeat kinase 2)-linked PD, may present clinically as "idiopathic PD" and account for at least 1% of all cases of PD. However, all known monogenic forms combined explain about only 20% of early-onset PD and less than 3% of late-onset PD at best. Although the individual clinical course cannot be predicted, overall, many cases of genetic PD will progress more slowly and respond better to treatment than patients without mutations. Genetic testing frequently yields inconclusive results, is expensive, and should be used for diagnostic purposes only after careful consideration in selected cases at specialty centers. While genetic findings have greatly advanced our understanding of the pathophysiology of PD, we are faced with many novel challenges. These include the definition of the phenotypic and genotypic spectrum of the monogenic forms, a revised terminology and classification of parkinsonian syndromes, identification of genetic susceptibility factors, and development of guidelines for genetic testing and of new treatment options for PD.