Over the course of his long and productive career, Euler wrote a thousand letters, and received almost two thousand more. Many of these letters have been published in various sources over the last two centuries, but many more have not appeared. To correct this difficulty, The Euler Archive has partly collected Euler's published correspondence into one location and made it available to everyone.
Series IV, Volume A of the Opera Omnia is almost completely devoted to Euler's correspondence. It lists the 2829 known letters to and from Euler, along with the dates they are written, information on where the original letter can be located, and summary (in German) of each letter. This is an invaluable resource, and a fascinating one.
Using some of the data provided in the Opera Omnia, along with our own historical research and library-scouring efforts, the Euler Archive has put together these correspondent pages. Here you can find information about Euler's major correspondents, along with information about where his correspondence can be found. In many cases, sections of his correspondence are available on the Euler Archive, and appropriate links are given.
The correspondence pages can be searched several ways. A list of all of Euler's correspondents, together with the numbers of letters exchanged, can be viewed several ways:
Bonus material: see P.H. Fuss's Correspondence, containing Euler's correspondence, an early index of Euler's works, and more.