Nonhuman animals can make relative quantity judgments of two or more sets of things (e.g., which set has more food items). However, a more sophisticated judgment requires assessing the larger or smaller set on the basis of a proportion within that set (e.g., red candies in a set of red and black candies). Judgments of relative proportion cannot rely on an absolute number comparison for the pertinent subset type but rather must rely on how much of one thing exists relative to another within each set. This more sophisticated form of quantitative cognition is nearly unstudied in other species. This thesis focused on the ability of capuchin monkeys and macaque monkeys to distinguish the relative proportions of stimulus sets presented to them on a computer screen. The task presented to capuchin and rhesus monkeys was to judge which of two matrices of varying numbers of items had the larger proportion of one designated color relative to the other. Rhesus monkeys performed above chance in all trial types and capuchins in all trials wherein they could rely on black dots as the relevant cue. Their performance was in accordance with Weber’s Law, indicating that the same mechanism may be at work when judging absolute and relative quantities.