L-Amino acids are predominant in living organisms, but D-amino acids such as D-alanine and D-glutamate also occur in all eubacterial cell walls. Moreover, even mammals contain endogeneous D-amino acids: D-serine functions as a signaling molecule in mammalian brains, and D-aspartate acts as a mediator in endocrine systems. Various other D-amino acids have been demonstrated in archaea, yeasts, fungi, plants, insects, mollusks and other eucaryotic organisms. These D-amino acids are mostly endogenous and produced in most cases by racemization from their corresponding antipodes by the action of racemases. Therefore, amino acid racemases play a central role in D-amino acid metabolism. Most amino acid racemases require pyridoxal 5'-phosphate (PLP) as a coenzyme, but several others require no coenzymes. Recently, the structures and functions of these two classes of amino acid racemases were clarified on a molecular basis. We here describe recent advances in studies of the functions and mechanisms of PLP-dependent and independent amino acid racemases.