This book constitutes the refereed proceedings of the 20th Australian Joint Conference on Artificial Intelligence, AI 2007, held in Gold Coast, Australia, in December 2007.The 58 revised full papers and 40 revised short papers presented together with the extended abstracts of 3 invited speeches were carefully reviewed and selected from 194 submissions. The papers are organized in topical sections on machine learning, neural networks, evolutionary computing, constraint satisfaction, satisfiability, automated reasoning, knowledge discovery, robotics, social intelligence, ontologies and semantic Web, natural language systems, knowledge representation, expert systems, applications of AI, and short papers.