Aby Warburg's Nachlass, the heart of the Warburg Institute Archive, is complemented by other large holdings which are no less remarkable. Quietly accumulating over the decades, still only provisionally cataloged, the vast corpus of letters filed as “General Correspondence” reveals itself to be a spectacularly rich resource for twentieth-century cultural and intellectual history. The secretariat was efficient: most everything was kept, letters received as well as copies of letters sent, meaning that the visitor to Woburn Square can sit in a single archive, read both sides of exchanges, and follow conversations over years — a rare circumstance. The academic staff maintained contact with an impressively large number of internationally active scholars, working in many fields, and the letters are characteristically substantive. In this article, the author indicates how a book she is presently writing, Warburg Circles, 1929 – 1964, has been shaped by these archival holdings. She offers a brief history of the consolidated archive now located on the fourth floor of the Warburg Institute, a story that throws light on recent scholarly trends; and she gives examples of the sorts of historical insight that may be gained from this corpus of correspondence.