For a long time D-amino acids were believed to be present only in prokaryotes. More recently, D-serine and D-amino acids were discovered in eukaryotes. In prokaryotes, D-serine is an important component of the cell wall. In some bacteria D-serine competes with β-alanine in the formation of coenzyme A. Thus, D-serine has to be eliminated in prokaryotes by degradation to pyruvate and ammonia using D-serine dehydratase, a fold- type II pyridoxal 5’phosphate-dependent enzyme, or by D-amino acid oxidases. D-Serine and other D-amino acids have been found essential in reactions of the endocrine and central nervous systems. D-Serine has been discovered in the human brain, with the highest concentrations found in those areas enriched in N-methyl-D-aspartate (NMDA) glutamate receptors, such as the forebrain. Robust receptor activation by glutamate occurs only if D-serine is present at the synaptic cleft; such D-serine serves as the coagonist (or the coactivator) to NMDA receptors. D-Serine is synthesised by serine racemases from L-serine. This review describes the purification, reaction mechanism and the three-dimensional structure of serine racemases, D-amino acid oxidases and D-serine dehydratases.