The physiology and pathology of membrane (contact or surface) digestion of the three main components of the food of animals and man is a rapidly developing field, and there is little doubt that newer ob servations will become available as time progresses. As the dis coverer of the phenomenon of membrane digestion, Professor Ugolev is well aware of the relative infancy of this subject, and in presenting the book at this time his primary purpose was to acquaint the reader with the experimental basis for the observed phenomenon and to stimulate interest in the further work required for the elucidation of details and for proper orientation. Translations as means of communication of ideas and experimental facts are of obvious importance and they require no apologies or explanations, particularly to those who are engaged in scientific endeavors. In making this book available to those who find the Russian language an insurmountable barrier, our main efforts were directed toward preserving the author's meaning, spirit, and intent as accurately as possible. It is hoped that this objective has been attained, and that the minor unintentional errors will be forgiven by the reader and by the author. The translator wishes to express to Professor Ugolev, whom he met in Leningrad in 1965, his appreciation for the trust in the ability of the translator to make Professor Ugolev's labor oflove, the present book, available in the English language."