This volume contains the proceedings of the 8th conference on Category Theory and Computer Science (CTCS '99) which was held in Edinburgh, UK, from September 10 to September 12, 1999.The purpose of the conference series is the advancement of the foundations of computing using the tools of category theory. While the emphasis is upon applications of category theory, it is recognized that the area is highly interdisciplinary.Previous meetings have been held in Guildford (Surrey), Edinburgh, Manchester, Paris, Amsterdam, Cambridge, and S. Margherita Ligure (Genova).Out of 39 submissions the Programme Committe has selected 19 for presentation at the Conference. The programme also included invited talks by Ryu Hasegawa (Tokyo), Peter Freyd (Pennsylvania), Marcelo Fiore (Sussex), Doug Smith (Kestrel Institute).We wish to thank the anonymous referees and the Programme Committee members: Jiri Adamek, Nick Benton, Rick Blute, Thierry Coquand, Martin Escardo, Masahito Hasegawa, Martin Hofmann (Chair), Peter O'Hearn, Dusko Pavlovic, Horst Reichel, Giuseppe Rosolini, Andre Scedrov.We also wish to thank the ENTCS editor Michael Mislove for encouragement and technical support.