This book is a textbook in basic category theory, written specifically to be read by researchers and students in computing science. The authors expound the constructions basic to category theory in the context of examples and applications to computing science. Some categorical ideas and constructions are already used heavily in computing sciences and many of these use are described. Other ideas, in particular the concept of adjoint have not appeared as widely in the computing science literature. The authors give an elementary exposition of those ideas they believe to be basic categorical tools, with pointers to possible application.

Michael Barr is Peter Redpath Professor in the Department of Mathematics and Statistics at McGill University in Montreal, Quebec. Charles Wells is Professor of Mathematics at Case Western Reserve University in Cleveland, Ohio.

This new edition contains all the material from the first and second editions, including the four chapters excised from the second edition and the solutions to all the exercises, as well as added material on factorization systems, monoidal categories, and other topics. All errors known to the authors have been corrected.