This article explores, through examination of correspondence and other primary sources, the close interaction between Henry Cowell and John Cage from 1933 to 1941 in the areas of percussion music, dance, world musics, the prepared piano, electronic sounds, micro-macrocosmic forms, sliding tones, and elastic composition. Several works are examined in detail, among them Cowell's Pulse (which anticipated Cage's micro-macrocosmic forms in the Constructions) and Cage's Imaginary Landscape No. 1 (whose electronic slides addressed Cowell's prediction that the “future of music” lay in the perfection of percussion and sliding tones). A previously unavailable recording of Imaginary Landscape No. 1 by Cage's ensemble reveals an unexpected interpretation of the score. Appendices present a chronology of events, a 1937 letter from Cowell to Cage, and a little-known set of Cage's program notes from 1940.