Pierre Boulez was a great letter writer and a frequent correspondent. Since the extent of his correspondence is vast and very little of it has been published in English, this chapter looks solely at Boulez’s epistolary exchanges with the composers Karlheinz Stockhausen, Olivier Messiaen, György Ligeti and Elliott Carter. While the correspondence with Stockhausen is one of the richest of all, only a brief sense of this can be given here. The correspondences have been selected on the basis that all four composers were pivotally important for Boulez in different ways. He had important friendships with them. He valued and performed their music and they in turn were fulsome in their appreciation of his championing their music as well as of his achievements as a composer. This brief consideration shows how Boulez not only pursued his own musical path but also promoted the music of his composer friends.