The scope of the 20th Mosbach Colloquium may be best illustrated by the following notes sent to the speakers when the colloquium was organized. "1) The application of inhibitors in cell biology has led to decisive insight into the organisation of cellular units. The subject should be treated against a background of current aspects of cell biology. In some areas of research, a fairly complete picture of the functions and cooperative interactions of the units has already emerged. We will discuss mainly these areas. 2) At this colloquium we want to contribute illustrations of the useful application of inhibitors to biological problems. Due to limited knowledge, inhibitors are sometimes incorrectly employed. This applies both to the planning of investigations and to the con· clusions drawn ("use of inhibitors by uninhibited workers"). 3) Inhibitors themselves are interesting substances and their mechanisms of action constitute fascinating problems." The colloquium has been subdivided into five sections. The known chemical structures of the inhibitors discussed are given in an Appendix. We gratefully appreciate the cooperation of the speakers. To a great extent, they managed the coordination of contributions to each of the five subdivisions. We regret that no contributors from Eastern Europe were able to participate.