D-Serine is a physiological coagonist of NMDA receptors involved in synaptic plasticity, neurodevelopment, and neurodegeneration. D-Serine is synthesized by the enzyme serine racemase, which converts L- to D-serine. Recent studies indicate that the supply of L-serine by astroglia fuels the neuronal synthesis of D-serine. This pathway, named the serine shuttle, highlights the importance of the glia–neuron metabolic crosstalk for regulating NMDA receptor activity. Dysfunction of different components of the serine shuttle pathway leads to neurodevelopmental defects, neurodegeneration, and may be involved in psychiatric diseases. Serine racemase and other components of the serine shuttle are therefore promising targets for neuroprotective drugs. Here we review several aspects of the neurobiology of D-serine focusing on mechanisms regulating D-serine signaling in health and disease.