Letters contained in the Correspondence series largely represent communication between Horvath and his colleagues, academic peers, and organizations of which he was a member, throughout his professional life. It is the largest series in the collection, spanning a period between 1948 and 1979. Early material includes correspondence between Dr. Horvath and the U.S. government, particularly members of the Allied Forces (post-WWII), the Department of Defense, the Navy, and the Air Force. Also included are invitations to and communications between various honor societies, including the Philosophical Society of Washington and the Cosmos Club.
The rest of the Correspondence series is largely comprised of papers containing discussions of research with colleagues, various personal letters to and from friends, conversations regarding the exchange of ideas, various requests for reprints of Dr. Horvath's papers, and requests for book and article reviews. Correspondents include James Gavin, Mervill Flood and Philip Morse.