This book describes the scientific collaboration of Milutin Milankovitch, academician and professor at University of Belgrade, and Wenceslas Jardetzky, professor at Columbia University in New York, presenting the chronological course of their work. It includes the famous Milankovitch's book "The Canon of Earth's Insolation and Its Application to the Problem of Ice Ages," and the ensuing scientific results of their collaboration, primarily through the presentation of their original correspondence over the course of three decades, from the 1930s to the 1960s. In addition to their scientific data, their questions, and their dilemmas that the correspondence abounds in, it is possible to find a wealth of other information, from the professional to the private, to information surrounding the general life circumstances of both scientists, as well as descriptions of the conditions in which they lived and worked before, during, and after World War II.