This book contains a selection of papers presented at the third annual workshop of the Esprit Working Group 21900 Types, which was held 12 - 16 June 1999 at Lokeberg in the rural area north of Goteborg and close to Marstrand. It was attended by 77 researchers. The two previous workshops of the working group were held in Aussois, France, in December 1996 and in Irsee, Germany, in March 1998. The proc- dings of those workshops appear as LNCS Vol. 1512 (edited by Christine Paulin- Mohring and Eduardo Gimenez) and LNCS Vol. 1657 (edited by Thorsten - tenkirch, Wolfgang Naraschewski, and Bernhard Reus). These workshops are, in turn, a continuation of the meetings organized in 1993, 1994, and 1995 under the auspices of the Esprit Basic Research Action 6453 Types for Proofs and Programs. Those proceedings were also published in the LNCS series, edited by Henk Barendregt and Tobias Nipkow (Vol. 806, 1993), by Peter Dybjer, Bengt Nordstrom, and Jan Smith (Vol. 996, 1994) and by Stefano Berardi and Mario Coppo (Vol. 1158, 1995). The Esprit BRA 6453 was a continuation of the former Esprit Action 3245 Logical Frameworks: - sign, Implementation and Experiments.The articles from the annual workshops organized under that Action were edited by Gerard Huet and Gordon Plotkin in the books Logical Frameworks and Logical Environments, both published by Cambridge University Press.

This book contains a selection of papers presented at the third annual workshop of the Esprit Working Group 21900 Types, which was held 12 - 16 June 1999 at Lokeberg in the rural area north of Goteborg and close to Marstrand. It was attended by 77 researchers. The two previous workshops of the working group were held in Aussois, France, in December 1996 and in Irsee, Germany, in March 1998. The proc- dings of those workshops appear as LNCS Vol. 1512 (edited by Christine Paulin- Mohring and Eduardo Gimenez) and LNCS Vol. 1657 (edited by Thorsten - tenkirch, Wolfgang Naraschewski, and Bernhard Reus). These workshops are, in turn, a continuation of the meetings organized in 1993, 1994, and 1995 under the auspices of the Esprit Basic Research Action 6453 Types for Proofs and Programs. Those proceedings were also published in the LNCS series, edited by Henk Barendregt and Tobias Nipkow (Vol. 806, 1993), by Peter Dybjer, Bengt Nordstrom, and Jan Smith (Vol. 996, 1994) and by Stefano Berardi and Mario Coppo (Vol. 1158, 1995). The Esprit BRA 6453 was a continuation of the former Esprit Action 3245 Logical Frameworks: - sign, Implementation and Experiments.The articles from the annual workshops organized under that Action were edited by Gerard Huet and Gordon Plotkin in the books Logical Frameworks and Logical Environments, both published by Cambridge University Press.