This volume is the issue of the symposium 'Theoretical Approaches to Complex systems', held in Tlibingen on June 11 and 12, 1977. The meeting was dedicated to the late Ernst Pfaffelhuber of the Institute for Information Sciences, University of Tlibingen, in memory of his con- tributions to information theory, physics, and biology and of his many fruitful contacts with the Tlibingen scientists gathered on this occa- sion. We would like to express our gratitude to all participants for their willingness to offer a weekend, for their help and the care they took in writing a contribution to this...