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Dementias that occur over weeks to months require a different differential than the slowly progressive dementias that occur over years. Because of the rapidity of decline in cognition, the evaluation of the patient is usually urgent and requires an extensive workup with multiple tests running in parallel. Creutzfeldt-Jakob disease, perhaps the prototypic rapidly progressive dementia (RPD), is often the first diagnosis many neurologists consider when faced with a patient with rapid cognitive decline. Many conditions other than prion disease, however, can present with RPD. Other etiologies include autoimmune conditions, cancers, uncommon presentations of other more common neurodegenerative dementias, and others. This chapter discusses some of the major etiologies for RPDs and offers an algorithm for diagnosis. Some topics, such as other neurodegenerative dementias and autoimmune dementias, are covered in other chapters in this issue, and will be mentioned here only briefly.