Frontotemporal dementias are syndromes of progressive dysfunction of the frontal and/or temporal lobes, either unilaterally or bilaterally. These syndromes were described clinically under the terms "primary progressive aphasia" in the United States and "frontotemporal dementia" in Europe and the United Kingdom. They are diagnosed by the clinical features of a frontal lobe neurobehavioral syndrome, or a language and cognitive deterioration. In recent years, molecular genetic findings in these syndromes, especially the tau and progranulin mutations on chromosome 17, have provided a molecular and genetic foundation for the understanding of frontotemporal dementia. These disorders are distinct from Alzheimer's disease but have some overlap with the syndrome of corticobasal degeneration, and with motor neuron disease. Treatments remain very limited, mainly involving therapy for the mood and behavioral symptoms, but advances in the molecular and genetic understanding of these conditions will hopefully lead to more specific therapies in the future.