His main interest was topology. He worked on the axiomatic treatment of homology with Norman Steenrod, and on homological algebra with Saunders Mac Lane, wrote a book on homological algebra with Henri Cartan that became a classic, and took part in the Bourbaki group meetings. Later in life he worked mainly in pure category theory. The *Eilenberg telescope* is a surprising construction, applying the telescoping cancellation idea to projective modules.

See also: Stefan Banach, Stanislaw Ulam.