Parkinsonism is a clinical syndrome, which is characterized by bradykinesia, rigidity, rest tremor, and postural instability. Idiopathic Parkinson disease (PD) is the most common cause of this syndrome, though there are several other important etiologies that must be considered. These include the atypical Parkinsonian disorders multiple system atrophy (MSA), dementia with Lewy Bodies (DLB), progressive supranuclear palsy (PSP), and corticobasal syndrome (CBS); as well as secondary causes of parkinsonism. These various disease entities may be distinguished based on key clinical features, which is critical for the purposes of diagnosis, treatment, and prognosis.