The most significant change was the introduction of body armor during the Xololpan phase (A.D. 450-650). Made of quilted cotton, this armor was as much as two or three inches thick and found in two basic types at Teotihuacan: one covered the entire body and limbs like mail, whereas the other was a type of tunic that extended only to the knees. This armor was effective against atlatl darts at a distance and could stop glancing or weak spear thrusts, but was not completely proof against darts cast off from close range or powerful spear thrusts. Nevertheless, it was very effective against slingstones which relied on impact rather than penetration, and this protection grew at atlatl fire forced slingers back to a greater and less effective distances. However, the armor was too bulky to wear on the march and was used primarily in set-piece battles rather than against ambushes or other guerilla-type actions. The earliest evidence of quilted cotton armor comes from Teotihuacan, but the city is located well above the cotton-growing regions, so the idea may have originated elsewhere even though it was adopted most widely at Teotihuacan. In any case, the use of armor required Teotihuacan to have access to, if not control of, cotton-producing areas, such as the agricultural areas of Morelos.

The armor must have been costly since the cotton had to be imported, spun, woven, and worked into a garment that consumed much more material than ordinary clothes. As a result, not all Teotihuacan soldiers could have been equipped with the costly armor, and most doubtless did without. The armor would have been worn by the troops in front, protecting these advancing elements and defending them against the breakup of combat formations: this was the attire of veterans and the elite who led in battle and bore the initial brunt of any clash. Quilted cotton helmets were also widely used, and elite examples were sumptuously decorated to indicate military rank or social status. Covering the head and nape of the neck, these were proof against slingstones and offered some protection against atlatl darts and shock weapons. The adoption of cotton body armor and helmets had a significant effect on combat practices, seen most clearly in the development of the shield.