This book constitutes the refereed proceedings of the 5th Internationa l Conference on Principles and Practice of Constraint Programming CPF9 9, held in Alexandria, Virginia, USA in October 1999. The 30 revised f ull papers presented together with three invited papers and eight post ers were carefully reviewed and selected for inclusion in the book fro m a total of 97 papers submitted. All current aspects of constraint pr ogramming and applications in various areas are addressed.