These notes, developed over a period of six years, were written for an eighteen lectures course in category theory. Although heavily based on Mac Lane's Categories for the Working Mathematician, the course was designed to be self-contained, drawing most of the examples from category theory itself.

The course was intended for post-graduate students in theoretical computer science at the Laboratory for Foundations of Computer Science, University of Edinburgh, but was attended by a varied audience. Most sections are a reasonable account of the material presented during the lectures, but some, most notably the sections on Lawvere theories, topoi and Kan extensions, are little more than a collection of definitions and facts.