This book constitutes the refereed proceedings of the 5th European Conference on Artificial Life, ECAL'99, held in Lausanne, Switzerland in September 1999. The 90 revised papers presented were carefully reviewed and selected from 150 submissions. The book presents the state of the art in ALife research. It is divided in topical sections on epistemology; evolutionary cybernetics; bio-inspired robotics and autonomous agents; self-replication, self-maintenance, and gene expression; societies and collective behavior; and communication and language.