To estimate the instructional effects of rewards or positive reinforcement, both primary and secondary, on classroom learning, 102 effect sizes were calculated from statistical data in 39 studies spanning a period of 20 years and containing a combined sample of 4,842 students in 202 classes. The mean of the studymedian effect sizes is 1.17, which suggests average percentiles on learning outcomes of 88 and SO, respectively, for positively reinforced and control groups. Contrary to previous theory and opinions, the strong effects of instructional reinforcement appear constant across grades (kindergarten through college), socioeconomic levels, race, private and public schools, and community types; but tangible reinforcers may be slightly more effective than intangible ones; and girls and students in special schools and classes may be slightly more affected by reinforcements than other students.W hat aspects of rewards or reinforcement are most effective for leaming? What techniques, on average, produce optimal results for different gender, race, and socioeconomic-status groups of students? The purpose of this paper is to provide a quantitative summary of recent research relevant to these and other questions by using statistical techniques developed by Glass (1977), Light and Smith (1971), Rosenthal (1976), and others to synthesize studies within a research domain . The present synthesis con cerns experimental and quasi-experimental research in natural classroom settings.The idea that learning can be initiated, stimulated, or increased by conscious use of rewards is not a twentiethcentury discovery. Aristotle and Plato argued that learners mus t be stimulated or motivated to awaken interest and desire for learning. Birnbaum (1962) describes how edible re inforcements (nuts , figs, and honey) were used as rewards in the twelfth-cen tury teaching of the Torah (p. 385). Birnbaum (1975, p. 225) also cites medieval manuscripts teaching parents and guardians about the use of educational re inforcements-for example , immediate and delayed reinforcements as well as social , tangible, and other types -to in duce children to acquire study habits and to love learning. In these writings , the developmental aspects of reinforcement are clearly described; and primary and secondary reinforcements such as food and praise are both exemplified.Classic learning theorists of the current century (Watson