This classic work is now available in an unabridged paperback edition. Hilton and Wu's unique approach brings the reader from the elements of linear algebra past the frontier of homological algebra. They describe a number of different algebraic domains, then emphasize the similarities and differences between them, employing the terminology of categories and functors. Exposition begins with set theory and group theory, and continues with coverage categories, functors, natural transformations, and duality, and closes with discussion of the two most fundamental derived functors of homological algebra, Ext and Tor.