Parkinson disease (PD) is a multifactorial neurodegenerative disease that was long considered the result of environmental factors. In the past 15 years, however, a genetic aetiology for PD has begun to emerge. Here, we review results from linkage and next-generation sequencing studies of familial parkinsonism, as well as candidate gene and genome-wide association findings in sporadic PD. In these studies, many of the genetic findings overlap, despite different designs and study populations, highlighting novel therapeutic targets. The molecular results delineate a sequence of pathological events whereby deficits in synaptic exocytosis and endocytosis, endosomal trafficking, lysosome-mediated autophagy and mitochondrial maintenance increase susceptibility to PD. These discoveries provide the rationale, molecular insight and research tools to develop neuroprotective and disease-modifying therapies.