The N-methyl-D-aspartate (NMDA) receptors play key roles in excitatory neurotransmission and are involved in several important processes, including learning, behavior, and synaptic plasticity. The regulation of NMDA receptor neurotransmission has been extensively studied, but many important questions still remain unsolved. One of the most debated aspects of the NMDA receptor regulation relates to the identity, role, and cellular origin of the NMDA coagonist(s). In addition to glutamate, the NMDA receptor activity was believed to be regulated by the coagonist glycine. More recently, D-serine has also been proposed to play a role as a key coagonist for NMDA receptor activity and neurotoxicity. A surprising unique biosynthetic pathway for D-serine has been demonstrated, indicating the conservation of D-amino acid metabolism in mammals. D-Serine was originally shown to be exclusively made in astrocytes, indicating a possible role as a gliotransmitter. Nevertheless, recent data indicate that D-serine has a neuronal origin as well, which raises several new questions on D-serine disposition. In this review, I discuss recent advances in the field and propose a novel model of D-serine signaling that includes a bidirectional flow of D-serine between astrocytes and neurons.