It is generally accepted that progressive, irreversible and regionally specific neurodegeneration and the presence of Lewy bodies are the essential pathological hallmarks of idiopathic parkinsonism. The causes of these phenomena, however, remain to be elucidated. One of the leading hypotheses is that oxidative stress induced by reactive oxygen species (ROS), such as the hydroxyl radical, damages essential components of the neuron, resulting ultimately in cell death. Observations in the parkinsonian brain at post-mortem support this hypothesis; for example, widespread oxidative protein modification is evident. There are several potential sources of increased oxidative stress in Parkinson's disease, including mitochondrial dysfunction, increased free iron levels and impaired free radical defence mechanisms. Further, it is possible that glial, rather than neuronal, elements are primarily responsible for the initial increase in oxidative stress in the substantia nigra. It is likely that parkinsonism is the result of aberrations at multiple levels of neuronal function. Oxidative stress is no doubt one of the events involved in neurodegeneration, but is unlikely to be the initiating event. It is to be expected that the search for this event will continue for many years.