ABSTRACT
Postnikov systems), homological algebra (e.g., the cohomology of groups), category theory, and simplicial sets. Of these ideas, category theory has had the widest appeal, and I have had many opportunities to lecture on categories, including the following occasions: the 1964 Colloquium Lectures of the American Mathematical Society; at Heidelberg University with support by the Alexander von Humboldt Foundation; at Kings College London on invitation from Professor A. Frohlich; at St. Andrews in Scotland; at Canberra, Australia; at Bowdoin College during lectures sponsored by Dan Christy and the National Science Foundation; and at Tulane University, thanks to Paul Mostert and the Ford Foundation.
