Transfer ribonucleic acids (tRNAs) occur in all living organisms. In biological protein synthesis they accept activated amino acids which are then transferred to growing peptide chains. With molecular weights lying between 25000 and 30000, tRNAs are easily within the reach of today's physical, chemical, and biochemical methods. The primary structures of several tRNAs as well as some relationships between structure and function have been elucidated. Three‐dimensional structure, specificity, and mechanism of action are the subjects of present research efforts. Copyright © 1969 by Verlag Chemie, GmbH, Germany