This paper reviews several issues regarding one- versus two-factor theories of learning. First, the traditional distinctions between classical and operant conditioning are presented. This includes both theoretical and experimental contrasts. Second, empirical evidence in support for a one-factor theory is examined. Numerous research studies involving operant conditioning of the autonomic nervous system are discussed. The basic hypothesis tested in the studies is that involuntary responses may be demonstrated to come under control of operant reinforcers. Thus, the need for only one kind of learning is demonstrable. The studies involve operant conditioning of heart rate, blood pressure, galvanic skin response, brain waves, etc., with both animal and human subjects. The vast majority of the experiments indicate significant results in favor of operant conditioning of the autonomic nervous system. Critics' questions and evaluations of such findings are reported. These generally surround the usage of yoked control design, conditioning versus control mediation. Lastly, the practical significance of experimental findings which suggest a one-factor theory are explored. (Author)