Primary progressive aphasia (PPA), typically resulting from a neurodegenerative disease such as frontotemporal lobar degeneration or Alzheimer's disease, is characterized by a progressive loss of specific language functions with relative sparing of other cognitive domains. Three variants of PPA are now recognized: semantic variant, logopenic variant, and nonfluent/agrammatic variant. We discuss recent work characterizing the neurolinguistic, neuropsychological, imaging and pathologic profiles associated with these variants. Improved reliability of diagnoses will be increasingly important as trials for etiology-specific treatments become available. We also discuss the implications of these syndromes for theories of language function.