This volume contains the complete correspondence of two of the leading analytic philosophers of the late twentieth century, David Armstrong and David Lewis. Comprising some 345 letters, including letters to and from third parties, this correspondence is at once a deep philosophical resource shedding new light on the philosophical development of two of the great late twentieth-century philosophers, and a record of the development of a philosophical friendship. The letters are valuable, not only for the light they shed on the background to many of Armstrong's and Lewis' major publications, but also for what they show us about the practice of philosophy. The correspondence is a window into both the philosophical content of two leading thinkers' minds and an exemplar of how philosophy of the highest level can be done.