Arboreal mammals have longer, heavier tails than closely related terrestrial counterparts, a pattern established independently across several clades. Functionally, relatively massive tails are thought to promote stability by serving as dynamic regulators of angular momentum during precarious arboreal travel. However, empirical demonstrations of this assumed biological role remain rare. Here, we use kinematic data on primate locomotion from both the field and the lab to investigate the biomechanical function of tail usage during arboreal quadrupedalism. High-speed video analyses of 11 species of platyrrhines moving in their natural habitat show that wild arboreal primates modulate tail position in response to variation in substrate diameter and orientation (precisely measured with remote sensing). On narrower and more inclined substrates, monkeys use significantly more extended tail postures and increase overall tail displacement, consistent with the dynamic use of the tail to control whole-body angular momentum and locomotor stability. Quantitative measurements of segmental angular momenta in captive squirrel monkeys (Saimiri boliviensis) moving on narrow perches support this interpretation. Squirrel monkeys consistently hold their tails in extended positions throughout the gait cycle and use the large rotational inertia of the extended tail to effectively dampen trunk angular momentum (i.e., angular momentum cancellation = 89.5-95.4%). Collectively, these studies provide empirical support for the hypothesis that relatively long, massive tails have evolved as stabilizing organs among arboreal animals. Supported by NSF BCS-1126790, BCS-1640453, BCS-1640552, and BCS-1921314