In this review we discuss the epidemiological, clinical, and genetic characteristics of early-onset parkinsonism, defined as parkinsonism starting before age 40 (sometimes 50) years. Juvenile parkinsonism is very rare and is the result of various secondary or genetic causes. In patients with onset at or above age 21 years, secondary causes require exclusion but are rare; most cases with a fairly pure parkinsonian syndrome (eg, young-onset Parkinson's disease; YOPD) are due to typical Lewy-body Parkinson's disease or, less commonly, genetic causes. In comparison with patients with late-onset disease, most patients with YOPD progress more slowly in terms of motor features and have a longer disease course with preservation of cognitive function, but typically develop motor fluctuations and dyskinesias earlier. Patients with YOPD generally experience a greater effect in their lives than those with late onset, with poorer social adjustment, higher rates of depression, and lower quality of life. Management of YOPD must therefore aim to maintain occupational, social, and daily functioning, while delaying or ameliorating motor complications of treatment, providing psychological support, and, where possible, preventing psychiatric complications including depression.