Whilst specific examples of functors and natural transformations had been given by Samuel Eilenberg and Saunders Mac Lane in a 1942 paper on group theory, these concepts were introduced in a more general sense, together with the additional notion of categories, in a 1945 paper by the same authors (who discussed applications of category theory to the field of algebraic topology). Their work was an important part of the transition from intuitive and geometric homology to homological algebra, Eilenberg and Mac Lane later writing that their goal was to understand natural transformations, which first required the definition of functors, then categories.
whilst specific examples of functors and natural transformations had been given by samuel eilenberg and saunders mac lane in a 1942 paper on group theory these concepts were introduced in a more general sense together with the additional notion of categories in a 1945 paper by the same authors (who discussed applications of category theory to the field of algebraic topology) their work was an important part of the transition from intuitive and geometric homology to homological algebra eilenberg and mac lane later writing that their goal was to understand natural transformations which first required the definition of functors then categories