Interactive radio instruction has gained attention as a low-cost means of improving the academic achievement of primary school students. The Radio Mathematics Project in Nicaragua and subsequent programs developed by the Learning Technologies for Basic Education (LearnTech) Project illustrate the evolution such programs have undergone as technology and society have changed. The term "interactivity," was used to characterize the simulated conversation of radio broadcasts in which students responded in chorus to questions posed by the radio teacher, but the concept has expanded. Programs started after Radio Mathematics, which was initiated in 1974, have presented new and different challenges. Concepts being taught are more complex, students are expected to apply what they learn outside the classroom, and classroom teachers have a more important role. Such programs as Radio Science, which is taught in Papua New Guinea, have employed new cognitive-learning theories. A new radio program based on constructivist principles is being used in South Africa. Radio projects have pioneered interactive instruction and remain in the vanguard of the application of learning theory to education in the developing countries. (Contains 20 references.) (SLD)