Although the phonological innovations that derive Arapaho from Proto-Algonquian are extensive, the historical origins of Arapaho inflectional morphology can be traced in almost all cases. Some new morphological categories and patterns have emerged, and some functions of inherited material have changed. The data are drawn from grammars, early texts, and dictionaries, supplemented by sound recordings and fieldwork. The examination of data from these varied sources in the context of historical analysis sheds light on synchronic Arapaho phonology, including pitch accent, tone, and syllabification. Improved transcriptions of paradigmatically related forms make possible an historical explanation of the major accentual patterns.