The American Songbook has been a fruitful source of improvisation for jazz musicians, either through artists interpreting those songs themselves, or crafting new songs from their chord changes as bebop musicians did prolifically in the 1940s. This chapter investigates this influence, beginning with the debt that jazz improvisers owe to Tin Pan Alley composers, before turning that relationship around to consider how the success of those same songwriters depended on an ongoing attempt to identify what made jazz appealing to American listeners and distil aspects of that enigmatic essence into the commercially viable object of popular song. In examples like Harold Arlen’s “Stormy Weather,” Irving Berlin’s “Putting on the Ritz,” or any number of Cole Porter compositions, we see the workmanlike creators of Tin Pan Alley incorporating not just musical elements associated with jazz, but also a more general “sensibility,” intended to recreate the music’s blues-informed world-weariness or performative impertinence.