This book constitutes the refereed proceedings of the 6th European Conference on Artificial Life, ECAL 2001, held in Prague, Czech Republic, in September 2001. The 54 revised papers and 25 posters presented together with five invited papers were carefully reviewed and selected from numerous submissions. The book reflects the state of the art in ALife. It is divided into topical sections on agents in environments; artificial chemistry; cellular and neural systems; collaborative systems; evolution; robotics; vision, visualization, language, and communication; and miscellaneous.