There is no doubt about the importance of hydration in many areas of every day life, technology, biology, medicine, science etc. During the last years many investigations have been carried out upon problems of hydration and a large amount of experimental and theoretical data has been obtained by the application of different methods. One effi­ cient possibility to stimulate progress in scientific problems is to come together and discuss existing results and ideas. This was the aim of the 35th Bunsenkolloquium and a subsequent seminar held in Marburg, FRG from April 2 -4, 1987 with respect to the "Interaction of Water in Ionic and Nonionic Hydrates". The meeting was attended by more than one hundred participants from 25 countries. It will be seen from the content of the chapters in this book, which comprises the introductory papers and more or less extended abstracts of research seminars, that it was possible not only to stress the advantages and disadvantages of each method, but also to show how information gained by one method can complement the results of another one in order to increase our overall understanding of hydration pheno­ mena. The papers are divided into sections concerning the hydration of: ions, nonionic substances, biological and macromolecular substances, surfactants as well as a section containing methods, models and theo­ ries, which may stimulate investigations on hydrations.