Euler’s papers and books are fundamental to every branch of the mathematical sciences. They are clearly and beautifully written, well explained and illustrated by examples. The Opera omnia belong in any major working library concerned with mathematics and its applications. The edition of the series I, II and III is limited, for the most part, to re-publication of works that Euler himself prepared for press. The texts are reprinted in the original language, most frequently French or Latin. The fourth series is devoted to Euler’s correspondence . Volume IV A 1, which has already been published in 1975, is a vademecum for the following nine volumes summarizing each of the more than 3000 letters and documents. Volume IV A 2 contains Euler’s correspondence with Johann I and Niklaus I Bernoulli (published in 1998), Volume IV A 5 Euler’s correspondence with Clairaut, d’Alembert and Lagrange (published in 1980), and Volume IV A 6 (published in 1986) contains the correspondence with Maupertuis and Frederick II. The remaining volumes deal with Euler’s correspondence with Daniel Bernoulli, Bertrand, Bonnet, Bouguer, Condorcet, Cramer, Delisle, Formey, Goldbach, Haller, Lalande, Lambert, Lesage, Lomosonov, Marinoni, Sanchez, Stanislav Auguste, Segner, Wettstein, and 260 others. The series IV B will contain Euler’s hitherto unpublished manuscripts, notebooks, and diaries.