This book constitutes the refereed proceedings of the 9th Portuguese Conference on Artificial Intelligence, EPIA '99, held in Évora, Portugal in September 1999. The 23 revised full papers presented together with three invited full papers were carefully reviewed and selected from a total of 66 submissions from 17 different countries. The papers are organized in topical sections on constraint programming, natural language processing, spatial reasoning, logic programming, theorem proving, reasoning under uncertainty, multi-agent systems, abduction and revision, and genetic algorithms.