Degeneration of dopaminergic neurons that project from substantia nigra to striatum is the primary mechanism that causes Parkinson disease (PD). This death of dopaminergic cells disturbs control over impulses sent from the motor cortex and hence results in the presence of three cardinal motor signs: tremor, rigidity, bradykinesia. The cause of Parkinson disease is unknown. Current treatments relieve symptoms but do not halt the progression of the disease. It is not yet known what causes neurons to degenerate. Influences of aging, environmental toxins, genetic susceptibility have been pointed out by researchers, but the theory of oxidative stress seems to be the most convincing. It is supposed that SN neurons are exposed to oxidative reactions from dopamine metabolism (production) during which hydrogen peroxide and toxic semiquinones are formed. Additionally, in brains of PD patients there are decreased concentrations of defence mechanisms such as glutathion and compensatory ferritin that binds iron, maintaining it in its safe state (Fe2+ iron takes part in Fenton reaction that leads to free radicals production). However, we have to admit that Parkinson disease is probably multifactorial, and the combination of the above stated factors may cause the disease.