In the early stage after the translation from mRNA, the proteins of the terrestrial organisms are composed of L-form amino acids (L-amino acids). The homochirality of the proteins derived from L-amino acids can be changed under the various conditions. L-Amino acid residues can epimerize in the proteins to become D-amino acid residues. L-Amino acids can also racemize as the free form. D-Amino acids have been detected from many kinds of organism and in the natural environments. This paper reviews on the examples of D-amino acid formation from organisms, the chemical mechanism of racemization and epimerization, and the action of D-amino acids as well as the hydrophobicity change from the epimerization in oligopeptides.