This volume presents the core of invited expository lectures given at the 1993 NATO ASI held at the University of York. The subject matter of the ASI was the interplay between automata, semigroups, formal languages and groups. The invited talks were of an introductory nature but at a high level and many reached the cutting edge of research in the area. The lectures were given to a mixed group of students and specialists and were designed to be accessible to a broad audience. The papers were written in a similar spirit in the hope that their readership will be as wide as possible. With one...