Research on the development of ability to control variables during adolescence is reviewed. The first section discusses studies by Piaget and his collaborators, replications of Piaget's studies, and related studies by other investigators. Factors which influence performance on controlling variables tasks are delineated; the existence of Piaget's stage of formal reasoning is considered. In the second section, attempts to train students to improve in ability to control variables are analyzed. These studies demonstrate that training improves logical thinking under some conditions. It appears that students need to learn to recognize and organize relevant information in addition to learning a particular strategy such as "make other things equal." Suggestions for future research and for development of educational programs are discussed. (Author)