Parkinson's disease (PD) is a progressive neurological disorder marked by nigrostriatal dopaminergic degeneration and development of cytoplasmic proteinaceous aggregates known as Lewy bodies. Although the pathogenic mechanisms responsible for PD are not completely understood, many clues have come from biochemical, epidemiological, and genetic studies. Mutations in certain genes found in rare, familial cases of PD, such as alpha-synuclein and parkin, suggest a role for the ubiquitin-proteosome system and aberrant protein aggregation. Biochemical analyses have implicated mitochondrial dysfunction in PD. Epidemiological and animal model studies point to a role for environmental toxins, some of which are mitochondrial inhibitors. Mitochondrial dysfunction, resulting from either genetic defects, environmental exposures or an interaction between the two, may cause alpha-synuclein aggregation or neurodegeneration through oxidative stress or excitotoxicity. A better understanding of the mechanisms underlying PD should reveal novel therapeutic targets.