Abstract: During evolution, proteins gradually accumulate large numbers of amino acid changes in their sequences. In distantly related proteins, therefore, it is difficult to deduce their evolutionary relationships by sequence comparisons. In such cases, the relationships may be uncovered by comparisons of three-dimensional protein structures, because during evolution the three-dimensional fold of polypeptide chain is better conserved than the constituting amino acids.The change-rates of amino acide residues depend on their locations in the three-dimensional structure; since the internal residues may be more important for protein stability than external ones, the amino acids change more easily on the surface than in the interior of protein molecule.Apart from the divergent evolution, there are two convergent evolutions, convergence to common function and convergence to similar structure. General rules for protein evolution obtained from currently available data are described here. [J. Cryst. Soc. Jpn, 28, 200 (1986) ] .