This study examines the development of action monitoring in adolescence by measuring the N200, the ERN (error-related negativity) and the P(E) (error positivity), which are event-related potentials (ERPs) that appear to be generated in the anterior cingulate cortex (ACC) and reflect action-monitoring processes. We predicted that amplitude would be significantly greater in late compared to early adolescence. Participants consisted of 11 adolescents that were divided into early (age 9-14) and late (age 14-17) adolescence groups. ERPs were recorded during an 840-trial arrow-flanker task and using 128-channel dense array EEG. Results indicated that there were no differences in P(E) amplitude but that N200 and ERN amplitudes were greater in the late adolescence group. According to the conflict monitoring hypothesis, this suggests that the ability to detect error-related conflict, which is involved in the modulation of cognitive control, appears to be fully developed later in adolescence and may be linked to the maturation of the ACC.