D-serine is a stereo-isomer of the common amino acid, L-serine. D-serine was only thought to exist in bacteria until relatively recently. D-serine was the second D amino acid discovered to naturally exist in humans. The first one was D-aspartate. D-serine is synthesized from L-serine by serine racemase (SRR), and it is degraded by D-amino acid oxidase (DAO). It is found in high abundance in the brain. D-serine acts on the glycine binding site of the N-methyl-D-aspartate receptor (NMDAR) and modulates glutamate-mediated receptor activation. For the receptor to open, glutamate and either glycine or D-serine must bind to it. In fact, D-serine is a more potent agonist at the glycine site on the NMDAR than glycine itself. The importance of D-serine in mammalian brain function is apparent from extensive investigations reported and reviewed over the past decade, including roles in synaptic plasticity and memory. D-serine is also implicated in the pathophysiology and therapy of several psychiatric and neurological conditions including schizophrenia and glioma. In schizophrenia, there is evidence that D-serine levels are decreased, a deficiency that may contribute to the proposed NMDAR hypofunction of the disorder and that has led to D-serine replenishment as a novel therapeutic strategy.