Feedback is highly contingent on behavior if it eventually becomes easy to predict, and weakly contingent on behavior if it remains difficult or impossible to predict even after learning is complete. Many studies have demonstrated that humans and nonhuman animals are highly sensitive to feedback contingency, but no known studies have examined how feedback contingency affects category learning, and current theories assign little or no importance to this variable. The following dissertation examined the effects of contingency degradation on rule-based and information-integration category learning. In addition to behavioral effects, these experiments constitute an initial investigation into the prefrontal cortex and the striatum as potential neurobiological mechanisms responsible for contingency detection or computation. Ultimately, learning was good in high-contingency conditions, but most participants showed little or no evidence of learning in low-contingency conditions. Possible causes of these effects are discussed, as well as their theoretical and neurobiological implications.