As an educational media, radio has a long and complex history in the United States, but its most extensive and important use in education took place during the School of the Air (SOA) movement. From approximately 1929 to 1975, commercial broadcast networks, state universities and departments or colleges of education, and local school boards operated over a dozen SOAs; one continued broadcasting into the mid-1990s. At their peak, SOAs reached approximately 2.5 million students nationwide and involved tens of thousands of teachers and children directly in radio broadcasting. They also generated scholarly studies by universities, government agencies, and foundations. Despite these achievements, educational scholars who mention SOAs dismiss them as failed experiments in educational technology. Their assessments are drawn from research conducted in the early 1940s, which concluded that despite high hopes, the SOA movement had never attracted more than 10% of the potential universe of K-12 student listeners. The author maintains that early scholars rushed to judgment, over-emphasizing the significance of audience studies/size, neglected to define any other criteria for measuring SOA success, and failed to document the accomplishments of the most successful SOAs such as NBC's Music Appreciation Hour, the Wisconsin School of the Air, and the Portland School of the Air. This article provides an overview of the origins of the SOAs movement and an in-depth look at several of the most successful SOAs, and demonstrates that in individual incidences, education by radio performed successfully for many decades, typically with niche audiences. The author hopes to fill gaps in knowledge and understanding of a significant in the history of American education and educational technology. (Contains 4 tables.)