This book constitutes the refereed proceedings of the 12th International Conference on Theorem Proving in Higher Order Logics, TPHOLs '99, held in Nice, France, in September 1999. The 20 revised full papers presented together with three invited contributions were carefully reviewed and selected from 35 papers submitted. All current aspects of higher order theorem proving, formal verification, and specification are discussed. Among the theorem provers evaluated are COQ, HOL, Isabelle, Isabelle/ZF, and OpenMath.