This volume presents the proceedings of the Symposium in Pure Mathematics held in New York City on April 10-11, 1968. The organizing committee felt that it was appropriate to devote attention to the applications of categorical algebra rather than to its autonomous development. It was explicit problems in topology and algebra which led to the engendering of category theory, and the applications continue to be numerous and lively. It is hoped the included papers show the diversity of research in categorical algebra.