The anterior cingulate cortex (ACC) participates in evaluating actions and outcomes. Little is known on how action-reward values are processed in ACC and if the context in which actions are performed influences this processing. In the present article, we report ACC unit activity of monkeys performing two tasks. The first task tested whether the encoding of reward values is co ntext dependent-that is, dependent on the size of theother rewards that are available in the current block of trials. The second task tested whether unexpected events signaling a change in reward are represented. We show that the context created by a block design (i.e., the context of possible alternative rewards) influences the encoding of reward values, even if no decision or choice is required. ACC activity encodes the relative and not absolute expected reward values. Moreover, cingulate activitysignals and evaluates when reward expectations are violated by unexpected stimuli, indicating reward gains or losses.