Genetic factors only account for up to a third of the cases of Parkinson's disease (PD), while the remaining cases are of unknown aetiology. Environmental exposures (such as pesticides or heavy metals) and the interaction with genetic susceptibility factors (summarized in the concept of impaired xenobiotic metabolism) are believed to play a major role in the mechanisms of neurodegeneration. Beside of the classical association studies (e.g. genome-wide association studies), a novel approach to investigate environmental risk factors are Mendelian randomisation studies. This review explores the gene-environment interaction and the gain of Mendelian randomisation studies in assessing causalities of modifiable risk factors for PD.