The orbitofrontal cortex (OFC) has been implicated in reinforcement-guided decision making, error monitoring, and the reversal of behavior in response to changing circumstances. The anterior cingulate cortex sulcus (ACCS ), however, has also been implicated in similar aspects of behavior. Dissociating the unique functions of these areas would improve our understanding of the decision-making process. The effect of selective OFC lesions on how monkeys usedthe history of reinforcementto guide choices of either particular actions or particular stimuli was studied and compared with the effects of ACCS lesions. Both lesions disrupted decision making, but their effects were differentiallymodulated bythe dependence on action– or stimulus–value contingencies. OFC lesions caused a deficitin stimulus but not action selection, whereas ACCS lesions hadthe opposite effect, disrupting action but not stimulus selection. Furthermore, OFC lesions that have previously been found to impair decision making when deterministic stimulus–reward contingencies are switched were found to cause a more general learning impairment in more naturalistic situations in which reward was stochastic. Both OFC and ACCS are essentialfor reinforcement-guided decision making ratherthan just error monitoring or behavioral reversal. The OFC and ACCS are both, however, more concerned with learning and making decisions, buttheir roles in selecting between stimulus and action values are distinct.