I am very glad to write a few introductory words to this important book by Professor Deryagin and his colleagues Drs. Krotova and Smilga. Ever since he first burst on the scientific scene in the 1930s, Professor Deryagin has continued to produce papers of seminal value in the general field of surface science, particularly those aspects of interest to physical chemists and colloid scientists. Many of the ideas introduced by Professor Deryagin and his long list of collaborators have entered into the broad stream of sci­ entific thought. Of course not all these ideas have been accepted and some have generated vigorous controversy. I am not sure that Professor Deryagin would have it otherwise for I sometimes have the feeling that he enjoys the cut and thrust of the scientific dia­ lectic. But none of these differences of opinion can detract from the quality of his work over many decades and from the impact of his contributions to surface science. In particular the ideas which he introduced with Landau (developed later but quite independently by Verwey and Overbeek in Holland) have laid the foundation for the whole of our present approach to colloid stability (the DLVO theory). A large part of Professor Deryagin's work has been con­ cerned with solid -liquid and liquid -liquid interactions. In the present book he and Drs.