Understanding the properties of NMDARs and their regulation by the local environment is of paramount importance for the sake of our central nervous system. The possibility that the glycine-site of NMDARs could represent an important pathway to regulate NMDAR activity was disregarded for a long time because based on the belief that this site was fully saturated by ambient glycine. We now know that glycine is not the only endogenous co-agonist of NMDARs and that both glycine and D-serine can differentially regulate the activity of these glutamatergic receptors. We will here review our latest results describing the key roles played by astroglial Ca2+, IP3 and membrane receptors in providing D-serine at excitatory synapses and how these processes impact synaptic plasticity and behavior in both a physiological and pathological context.