This book constitutes the refereed proceedings of the International Conference on Principles and Practice of Declarative Programming, PPDP'99, held in Paris, France, in September/October 1999. The 22 revised full papers presented together with three invited contributions were carefully reviewed and selected from a total of 52 full-length papers submitted. Among the topics covered are type theory; logics and logical methods in understanding, defining, integrating, and extending programming paradigms such as functional, logic, object-oriented, constraint, and concurrent programming; support for modularity; the use of logics in the design of program development tools; and development and implementation methods.