Neural mechanisms of cognitive control are hypothesized to support flexible, goal-directed behavior by representing task-relevant information in order to guide thought and action. The conflict monitoring theory proposes that anterior cingulate cortex (ACC) contributes to cognitive control by detecting conflicts in information processing and signaling when increased top-down control is required. This theory provides a computationally specified framework for understanding how cognitive control is recruited and explains a large literature of human neuroimaging studies reporting ACC activity in conditions of increased cognitive demand. Predictions from the theory have been tested and consistently confirmed in behavioral and neuroimaging experiments with human subjects. However, challenging findings from patients with ACC lesions and from studies of ACC function in nonhuman primates suggest that conflict monitoring may be just one facet of the broader role of ACC in performance monitoring and the optimization of behavior.