The 6th Kurt Gödel Colloquium was held from Wednesday, June 16th, to Saturday, June 19th, 1999, at the Casa de la Caritat, Barcelona. There were ten invited lectures held by international renowned researchers (Vincent Danos, Lou van den Dries, Mathew Foreman, Itsvan Juhasz, Byunghan Kim, Leonid Libkin, Angus Macintyre, Hiroakira Ono, Don Pigozzi, Jean Pierre Ressayre) and twentyfive contributed lectures.