The field of nucleic acids has grown to such a tremendeous size that it is impossible to include all publications concerning the chemistry and biological role of nucleic acids in an article of the length presented in this "Volume. Therefore, it is necessary to select the most important contributions and those not included "in well-known reviews. In many cases reference is made only to the authors who summarized their specialized field in chapters of the three volumes of "The Nucleic Acids" (edB. E. CHARGAFF and J. N. DAVIDSON, Acad. Press, New York 1955 and 19(0) or to the "Nucleic Acid Outlines" (V. R. POTTER, Burgess Publishing Comp. Minneapolis), where further literature and more detailed discussions may be found. Facts and theories will be dealt with, but not lists of references. Therefore it is not possible to follow in all cases the historical development of an idea and to admowledge all publications which might be important and inter esting from another point of view. Very little is mentioned about methods in the field of nucleic acids.