This monograph is neither a historical outline of the development of the concepts of protein biosynthesis and the structure and functions of the ribosomes, nor an exhaustive survey of the literature on these questions. The monograph is based upon an analysis of the modern trends in this field. The purpose of the monograph was to formulate more or less generalized representations of the structure and" function of the ribosome, as we envision it at the present day. It may be that this attempt is premature for a number of reasons, and the concepts outlined here will very soon be revised. Nonetheless, despite this risk, we believe it to be advisable to undertake this attempt for the following reasons: firstly, the undertaken analysis could aid in the comprehension of the substantial mass of extremely scattered experimental data on the ribosomes presently available; secondly, in any event, even if most of the concepts outlined rapidly become obselete, they can still serve as a stimulus for a whole series of experiments; and thirdly, we hope that some of the concepts outlined will still remain essentially correct and relatively stable. In view of the aforementioned, we should make the following reservations. First of all, we made no attempt to cite all the literature on the problems discussed, but considered it sufficient to illustrate the various premises with one or several sample references.