Among the great composers of American popular song in the “golden era” from 1920 to 1960, Harold Arlen (1905-86) is perhaps the least familiar by name. Although his classic songs like Over the Rainbow and Stormy Weather are widely known and admired, his compositional craft has been too little analyzed. Among the features to be discussed in this paper are Arlen’s creative and frequent transgression of the standard 32-bar, AABA song form, in long and lopsided structures that he referred to as tapeworms; the complex harmonic progressions and unusual inversions that keep tonality suspended and sometimes bring songs to end in keys different from where they began; and Arlen’s uncanny ability to create coherent melodic phrases that are often dissonant with the underlying harmonies.