These are the proceedings of the International Conference on Principles and Practice of Declarative Programming, PPDP'99, held in Paris, France, in September/October 1999. 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.