This comprehensive volume is a contribution to a new se ries initiated by the NATO Panel on "Gell to Gell Signals in Plants and Animals". The book reflects the outcome of an NATO work shop and bri ngs to mi nd two im portant questions: consideri ng the mass of relevant I iteratu reavai- able, is there any necessity for a new series of books - and considering the flood of compa rable meetings - is there any point in workshops of this nature and their publication? In order to deal with such questions adequately, much more space would be needed than is available in a foreword. Thus, the answers must remain rather superficial and, of course, rather subjective. To simplify the issue, the question of publication can be narrowed down to two fac tors - the financial risk, undertaken by the publisher, and the scientific risk, borne by the editor. If the book is good (with respect to lay-out and content) it will be a success - nothing will be lost the people involved will enhance their reputation! We are left with the question of the usefulness of workshops. Without doubt, it is indeed a useful procedure for experts to come together, in an atmosphere of harmony, and freedom from external pressures and time limitations, to discuss a well-defined theme. Wether in agreement or disagreement, a fair and open forum can be expectet for a variety of contributions.