Over the past few years, considerable progress has been made in understanding the molecular mechanisms of Parkinson disease (PD). Mutations in certain genes are found to cause monogenic forms of the disorder, with autosomal dominant or autosomal recessive inheritance. These genes include alpha-synuclein, parkin, PINK1, DJ-1, LRRK2, and ATP13A2. The monogenic variants are important tools in identifying cellular pathways that shed light on the pathogenesis of this disease. Certain common genetic variants are also likely to modulate the risk of PD. International collaborative studies and meta-analyses have identified common variants as genetic susceptibility risk/protective factors for sporadic PD.