The yaya corps who were originally the Turcoman peasants constituted the first regular army of the Ottoman state established in the age of Orhan Gazi. Acting as the hassa soldiery of the sovereign, the status of the corps had changed upon the establishment of the janissary garisons. Hence they were organized under a particular system called the ocak where they cultivated the land granted to them in compensation for their military service. The system shared certain similarities with the Byzantine organization of the stratiotes and the organization of the voynuks who were a group of soldiery preexisting in the Balkans before the Ottoman conquest of the area.Thus the study will mostly be concentrating on the particular condition of the yayas who served either as the peasants or the soldiers. On the other hand the müsellem corps who were a group of mounted troops of yaya origin appeared to be founded probably first in Rumelia. Both corps were reorganized as the auxiliary troops within the provincial soldiery by the second half of the fourteenth century. However, though they were no longer the hassa soldiery of the sovereign they continued to act as the active combatants in the major campaigns of the age.They appeared as the so called emergency troops recruited in times of nefîr-i ‘am, general call to arms in the first half of the fifteenth century. However, the corps began to be employed in the rear services by the second half of the fifteenth century untill the dissolution of their institution in 1582.Thus the study attempts to examine the corps in two successive stages. In the first stage the military importance of the yaya corps will be examined where in the latter the development of both corps as the auxiliary troops will be examined. Consequently the general aim of this study is to reveal the early stages of the corps.