As with many Ottoman institutions, the first half of the seventeenth century was a period of enormous change for the cavalry branch of the Ottoman standing army, or Alt? Bölük Halk?. Writers affiliated with the court complained of the infiltration of the organization by “outsiders” (ecnebi), men of peasant origin, whereas it had previously been largely exclusive to members of the sultan’s slave household, trained in the palaces of Istanbul and Edirne. Simultaneously, cavalrymen increasingly took up residence in provincial cities, where they assumed control over many aspects of the empire’s tax administration, transforming thereby into a wealthy, semi-hereditary elite. The organization became less exclusive in tandem with the expansion of its privileges and influence. In addition to the financial cost of maintaining this army, its members also threatened the state with rebellion on multiple occasions, ensuring that it would never be far from the attention of reformers. These developments, as well as the undoing of the cavalry army’s power following the reforms of Köprülü Mehmed Pasha (Grand Vizier 1656-61), have heretofore been understood through the often-biased writings of court officials and the retrospective accounts of Ottoman chroniclers. This paper turns instead to the organization’s surviving salary registers (mevâcib defterleri), introducing these key sources and the unique properties that enable them to be used to explore the cavalry army’s organizational, financial, and demographic history. The registers show how the corps’ regimental structure was reshaped by its members’ newfound power, permit a grounded analysis of the changing composition of its membership, and reveal how its old elite was replaced, under Köprülü Mehmed Pasha, by a new cadre of volunteers from across Anatolia and the Balkans. This archive-based approach allows us to move beyond the tropes of deterioration and corruption presented in the literary sources, producing as a result a more nuanced and multifaceted understanding of the cavalry corps’ transformation.