This book contains the proceedings of the 12th International Conference on TheoremProvinginHigherOrderLogics(TPHOLs'99), whichwasheldinNice at the University of Nice-Sophia Antipolis, September 14{17, 1999. Thirty- ve papers were submitted as completed research, and each of them was refereed by at least three reviewers appointed by the program committee. Twenty papers were selected for publication in this volume. Followingawell-establishedtraditioninthisseriesofconferences, anumberof researchers also came to discuss work in progress, using short talks and displays at a poster session. These papers are included in a supplementary proceedings volume. These supplementary proceedings take the form of a book published by INRIA in its series of research reports, under the following title: Theorem ProvinginHigherOrderLogics: EmergingTrends1999. The organizers were pleased that Dominique Bolignano, Arjeh Cohen, and Thomas Kropf accepted invitations to be guest speakers for TPHOLs'99. For several years, D. Bolignano has been the leader of the VIP team in the Dyade consortium between INRIA and Bull and is now at the head of a company Trusted Logic. His team has been concentrating on the use of formal methods for the e ective veri cationof securityproperties for protocols used in electronic commerce. A. Cohen has had a key in?uence on the development of computer algebra in The Netherlands and his contribution has been of particular imp- tance to researchersinterested in combining the severalknown methods of using computers to perform mathematical investigations. T. Kropf is an important actor in the Europe-wide project PROSPER, which aims to deliver the be- ts of mechanized formal analysis to system builders in industry.