This book constitutes the refereed proceedings of the 6th International Conference on Logic Programming and Automated Reasoning, LPAR'99, held in Tbilisi, Georgia in September 1999.The 23 revised full papers presented were carefully reviewed and selected from 44 submissions. Among the topics addressed are logic programming, constraint logic programming, complexity aspects, logical inference search, model checking, formal specification, rewriting, practical reasoning, Horn clauses, linear logic, partial evalutation, etc.