Rapidly progressive dementias are conditions that typically cause dementia over weeks or months. They are a particular challenge to neurologists as the differential diagnosis often is different from the more typical, slowly progressive dementias. Early and accurate diagnosis is essential, as many of the etiologies are treatable. The information in this review is in part based on experience through our rapidly progressive dementia program at the University of California San Francisco, Memory and Aging Center. As treatment of a rapidly progressive dementia is entirely dependent on the diagnosis, we present a comprehensive, structured, but pragmatic approach to diagnosis, including key clinical, laboratory, and radiologic features. For the 2 most common causes of rapid dementia, treatment algorithms for the autoimmune encephalopathies and symptomatic management for the neurodegenerative causes are discussed.