It has long been known that the impairment to discrimination learning caused by brief delays to reinforcement can be counteracted by the response-contingent presentation of a conditioned reinforcer during the delay interval following a correct response (Spence, 1947). More recently, it has been shown that reinforcement delay can also be overcome using response-marking procedures, in which the same stimulus contingently follows both correct responses and errors (e.g. Lieberman, McIntosh & Thomas, 1979). This thesis examined the effects of response-marking procedures on human learning of conditional discrimination tasks with delayed reinforcement. Experiments One to Three employed single case experimental designs (alternating treatments) to evaluate the effect of response marking during matching-to-sample tasks with delayed reinforcement, using children with autism as participants. Experiment One showed that for both marking and conditioned reinforcement supported acquisition of conditional discrimination performance over a 5 s delay, although the latter appeared more efficient. Experiment Two, however, showed that - with more effective techniques - both procedures were equally effective, and that both were more effective than a control in which no response-contingent stimuli occurred during the delay. Experiment Three compared the standard marking procedure with a novel marked-before procedure in which all sample stimuli were marked before a matching response was made. Both procedures produced very similar acquisition rates, and both were more effective in establishing conditional discrimination than a delay only control. Experiments four to Seven employed group comparison designs to compare marking against conditioned reinforcement, delay and immediate reinforcement using adult humans in a laboratory version of the matching-to-sample task. Marking effects were found only in Experiment Seven, when the confounding effects of verbal behaviour were adequately controlled. Overall, the findings indicated that response-marking procedures may be effective with human participants but that their effects are more reliable in applied settings with children than in laboratory settings with adults.