Parkinson's disease is the second most common neurodegenerative disorder. Despite fairly typical clinical and pathologic features, the etiology remains unknown. Many cellular mechanisms such as oxidative stress, mitochondrial dysfunction, lysosomal dysfunction, neuroinflammatory changes, and the formation of pathologic inclusions have been proposed as potential causes. Potential links between environmental and genetic changes appear to predispose individuals to develop Parkinson's disease. Considering these observations, albeit with different levels of evidence, it is becoming more probable that Parkinson's disease is a heterogeneous disorder or a syndrome that arises from the contribution of many different factors.