Homological algebra had its origins in the 19th century, via the work of Riemann (1857) and Betti (1871) on "homology numbers," and the rigorous development of the notion of homology numbers by Poincaré in 1895. A 1925 observation of Emmy Noether [N25] shifted the attention to the "homology groups" of a space, and algebraic techniques were developed for computational purposes in the 1930's. Yet homology remained a part of the realm of topology until about 1945.During the period 1940-1955, these topologically-motivated techniques for computing homology were applied to define and explore the homology and cohomology of several algebraic systems: Tor and Ext for abelian groups, homology and cohomology of groups and Lie algebras, and the cohomology of associative algebras. In addition, Leray introduced sheaves, sheaf cohomology and spectral sequences.At this point Cartan and Eilenberg's book [CE] crystallized and redirected the field completely. Their systematic use of derived functors, defined via projective and injective resolutions of modules, united all the previously disparate homology theories. It was a true revolution in mathematics, and as such it was also a new beginning. The search for a general setting for derived functors led to the notion of abelian categories, and the search for nontrivial examples of projective modules led to the rise of algebraic K-theory. Homological algebra was here to stay.Several new fields of study grew out of the Cartan-Eilenberg Revolution. The importance of regular local rings in algebra grew out of results obtained by homological methods in the late 1950's. The study of injective resolutions led to Grothendieck's theory of sheaf cohomology, the discovery of Gorenstein rings and Local Duality in both ring theory and algebraic geometry. In turn, cohomological methods played a key role in Grothendieck's rewriting of the foundations of algebraic geometry, including the development of derived categories. Number theory was infused with new results from Galois cohomology, which in turn led to the development ofétale cohomology and the eventual solution of the Weil Conjectures by Deligne.Simplicial methods were introduced in the 1950's by Dold, Kan, Moore and Puppe. They led to the rise of homotopical algebra and nonabelian derived functors in the 1960's. Among its many applications, perhaps André-Quillen homology for commutative rings and higher algebraic K-theory are the most noteworthy. Simplicial methods also played a more recent role in the development of Hochschild homology, topological Hochschild homology and cyclic homology.This completes a quick overview of the history we shall discuss in this article. Now let us turn to the beginnings of the subject.