In protein engineering, wherein the goat is desirable function and high conformational stability, the characteristics of each constituent amino acid residue are important, in terms of the overall characteristics of the target protein. Bovine pancreatic riobonuclease A (RNase A) is, historically, one of the most intensively analyzed proteins, and a considerable amount of information is available on amino acid-level information. Such data would serve to aid the understanding of relationships between the distribution of various amino acid residues in the protein molecule and the unique structure and/or functions of RNase A. This review summarizes the thus-far clarified roles of 38 of the total 124 amino acid residues which comprise RNase A, with respect to protein function, stability, and folding.