Parkinson's disease (PD) is one of the most frequent progressive neurodegenerative diseases in the elderly. This chapter reviews the recent developments in epidemiologic research of PD. It discusses the risk factors, diagnosis, disease frequency, prognosis, intervention, and implications for clinical practice. Age is the most important risk factor for PD. Smoking is one of the determinants of PD that has been studied most frequently. For the diagnosis of PD, the postmortem neuropathologic examination of the brain is generally considered to be the gold standard. For the clinical diagnosis of PD, the generally accepted prerequisite is assessment of the presence of Parkinsonism, based on a combination of four typical clinical features, referred to as cardinal signs, that is, resting tremor, bradykinesia, rigidity, and impaired postural reflexes. The rate of deterioration of nigral cells is probably most prominent and curvilinear in the preclinical or very early phase of Parkinson's disease.