This study deals with the different types of the rank of optio in the Roman Imperial army. While administrative, supra-centurial optiones were counted amongst the immunes [o. carceris, o. fabricae, o. valetudinarii, o. plumbariorum], the optio centuriae, who is usually thought of, when an optio is mentioned, belonged to the principales and held the middle one of the three tactical ranks. Similar to the tesserarius and the signifer, he was assigned to a contubernium and had no rooms of his own. The optio spei was immediately subordinate to the centurio and was the only kind of optio who could reasonably expect to be promoted to that position. Optiones supported and deputised for their centurio, commanded vexillationes, and helped with organising food supplies. Most problematic as to rank and tasks is the optio equitum who is attested for some mounted units beside a duplicarius. In artworks the staff [hastile] is the sign of an optio. In larger military units soldiers of equal rank formed several associations, while in smaller ones all principales formed a single association. From the mid-3rd century onwards, the comparable rank of magister coexisted with the optio.