This study essentially questions the nature of the yörük population in Ottoman western Thrace through an examination of fiscal and military registers of the sixteenth century. Firstly, through a discussion of the terminology used for mobile social groups in other disciplines, such as anthropology and geography, it is revealed that there is no terminological consensus, but rather a variety within and among the terms used. In parallel with this theoretical background, it is argued that the concept of variety occupied an important role in yörüks’ ways of life in the region in question. This is also reflected in the manners in which they were registered, though a different manner of registration did not necessarily signify a different way of life. In this particular sphere, the yörük groups examined are those of the districts of Demürhisar, Drama, Yenice-i Karasu, and Gümülcine. Next, the military nature of these yörüks is analyzed. The origins and formation of the yörük organization in Rumelia are discussed through the case of western Thrace. Similarities with and differences from other auxiliary forces are also investigated in connection with the question of origins and formation. The regulations issued for the yörüks as a military group and the changes that occurred over time are looked at so as to be able to see any differentiation. The extent of militarization among the yörüks in the region is evaluated through the classical fiscal surveys and the yörük registers, with a revisional approach to the literature being taken. Within this scope, the nature of the yörük registers is questioned, and the correlation between registered and unregistered yörüks is revealed.