The mammalian brain contains unusually high levels of d-serine, a d-amino acid previously thought to be restricted to some bacteria and insects. In the last few years, studies from several groups have demonstrated that d-serine is a physiological co-agonist of the N-methyl d-aspartate (NMDA) type of glutamate receptor – a key excitatory neurotransmitter receptor in the brain. d-Serine binds with high affinity to a co-agonist site at the NMDA receptors and, along with glutamate, mediates several important physiological and pathological processes, including NMDA receptor transmission, synaptic plasticity and neurotoxicity. In recent years, biosynthetic, degradative and release pathways for d-serine have been identified, indicating that d-serine may function as a transmitter. At first, d-serine was described in astrocytes, a class of glial cells that ensheathes neurons and release several transmitters that modulate neurotransmission. This led to the notion that d-serine is a glia-derived transmitter (or gliotransmitter). However, recent data indicate that serine racemase, the d-serine biosynthetic enzyme, is widely expressed in neurons of the brain, suggesting that d-serine also has a neuronal origin. We now review these findings, focusing on recent questions regarding the roles of glia versus neurons in d-serine signaling.