  After their 1945 paper, it was not clear that the concepts of category theory would amount to more than a convenient language; this indeed was the status quo for about fifteen years. Category theory was employed in this manner by Eilenberg &amp; Steenrod (1952), in an influential book on the foundations of algebraic topology, and by Cartan &amp; Eilenberg (1956), in a ground breaking book on homological algebra. (Curiously, although Eilenberg &amp; Steenrod defined categories, Cartan &amp; Eilenberg simply assumed them!) These books allowed new generations of mathematicians to learn algebraic topology and homological algebra directly in the categorical language, and to master the method of diagrams. Indeed, without the method of diagram chasing, many results in these two books seem inconceivable, or at the very least would have required a considerably more intricate presentation.