Peroxisomes (microbodies) have a long evolutionary lifespan, but in cell biology they are still young. It is gratifying to see that most recent advances in our knowledge of peroxisomes come from quite different disciplines, ranging from molecular biology and cell biology to clinical research. The identification in terms of cDNA sequences of enzymes, the localization in terms of immuno-electron microscopy, and the cell biological aspects of the organelle biogenesis have seen substantial progress very recently. Likewise, the phenomenon of peroxisome proliferation induced by a number of drugs, e. g. hypolipidemics and plasticizers, constitute an active area of research, where much is still to be learnt. The medical problem of inborn errors of metabolism seems to have gained a new realm for discovery. Here, the advances in knowledge within the last year have been particularly striking. Therefore, the "International Symposium on Peroxisomes in Biology and Medicine", held in Heidelberg from July 14-17,1986, was even more excit- ing than could have initially been anticipated. The present volume consists of papers from that conference. For sake of rapid publication, the camera- ready format, inspite of all its drawbacks (e. g. no proof reading by authors), was chosen. It is our pleasure to acknowledge support and help. The Symposium was supported by Deutsche Forschungsgemeinschaft, Bonn, by the Ministerium ftir Wissenschaft und Kunst Baden-Wtirttemberg, Stuttgart, as well as by a number of generous contributors from industry.