The 'psychological refractory period' has come to be the accepted label for response-delay phenomena in a double-stimulation situation. In the classic arrangement the two stimuli S1 and S2, requiring the two responses R1 and R2, are presented at interstimulus intervals of less than 500 msec. With decreasing interstimulus intervals the latency of R2 is known to increase steadily. The early studies on the refractory period were primarily concerned with the probability aspects of the various parameters involved. These parametric studies have produced a wealth of results that can hardly be encompassed by any one theory. Among the various theories that have been proposed over the years, three are most likely to survive: singlechannel, expectancy, and response-conflict theory. Single-channel theory, first proposed by Craik (1948), explains the delay in R2 in terms of a limited capacity of the central processing system (single-channel). The single channel can process only one stimulus at a time. Under double or sequential stimulation, signals are either queued or lost if the pace of their temporal presentation exceeds the basic processing time of the channel. According to this assumption, a delay in R2 should occur if the interstimulus interval falls below a critical value. In contrast to the single-channel notion, Poulton (1950) attributed delays in R2 at short interstimulus intervals to the fact that the subject