The present study examined how the magnitude of time-out duration following incorrect responses affected the ability of adults to learn simple visual discriminations. Sixty-four college students were randomly assigned to one of four groups that received a 0-, 5-, 10-, or 20-s time-out duration after an incorrect response. Each participant completed a simple visual discrimination task by attempting to determine and remember the correct response for each of eight different color-shape combinations. The results indicated that timeout was effective at enhancing performance, even when reinforcers for correct responses were used. In addition, the effectiveness was dependent on the duration of the time-out. Punishment can be defined as a consequence of behavior that reduces the future probability of that behavior (Azrin & Holz, 1966). Punishment is typically either positive or negative. Positive punishment involves the application of an aversive stimulus, after an undesirable behavior, that results in the reduction of the probability of recurrence of that behavior. For example, when rats were punished with a strong shock for pressing a lever that delivered food, the rats rarely pressed the lever for food (Camp, Raymond, & Church, 1967). Negative punishment involves the removal of a desired stimulus following an undesirable behavior that results in the reduction of the probability of the recurrence of that behavior (