Abnormal behavior commonly occurs in captive nonhuman primates. It differs from normal behavior in either kind or degree and consists of behaviors such as pacing, self-grasping, and self-injurious behavior. The extent of abnormal behavior in a population can be impacted by variables such as the animals’ sex, species, and age; however, environmental risk factors such as nursery rearing, single housing, and veterinary procedures also play a role. Self-injurious behavior (SIB) is of particular concern due to a greater potential for wounding. Although SIB has been referred to as a form of self-directed aggression, stress, rather than aggression, may play a greater role; SIB may actually be utilized as a means to reduce stress. Wounding occurs in a small percentage of the animals exhibiting SIB and may be associated with the animal’s inability to inhibit certain behaviors. Proper rearing, environmental enrichment, and social housing can help reduce abnormal behavior, but understanding the risk factors with an aim of prevention is the best strategy.