At the annual meetings ofthe "Fast Reactions in Solution Discussion Group" of the Royal Society of Chemistry, an increasing number of contributions is concerned with reactions in complex liquids, where the solvents cannot be regarded as homogeneous media but where their microstructure has to be taken into account. In order to summarize the different aspects of those solvents, the 1988 meeting of the group has been held as a symposium devoted to "Compartmentalized Liquids". The contributions concerned different fields of science from mathematics, physics, and chemistry to food research and pharmacy. Thus it was appropriate to organize the meeting afthe Zentrum fUr interdiszipliniire Forschung at Bielefeld. Envisaging compartmentalized liquids on a scale descending size, we may order the systems described in this volume from colloidal solutions, gels, hydro col­ loids, membranes, vesicles, microemulsions, micellar solutions, macrocyclic complexes to "cages" involved in bimolecular encounter. In recent years many experimental and theoretical studies have been performed on those systems, and they are also of growing interest for technical applications. Two important reasons for the investigation of compartmentalized liquids as reaction media are: i) they are suited for performing reactions between water-soluble and oil-soluble reactants, ii) they provide large internal interfaces, where reactions may be catalyzed. Due to the large interfaces, the diffusion of molecules is restricted, and for the theoretical treatment of reaction rates in these systems the concept offractal geometry seems to be appropriate. Therefore contribu­ tions concerning fractals are included in this volume.