New neurons are continuously generated from resident pools of neural stem and precursor cells (NSPCs) in the adult brain. There are multiple pathways through which adult neurogenesis is regulated, and here we review the role of the N-methyl-D-aspartate receptor (NMDAR) in regulating the proliferation of NSPCs in the adult hippocampus. Hippocampal-dependent learning tasks, enriched environments, running, and activity-dependent synaptic plasticity, all potently up-regulate hippocampal NSPC proliferation. We first consider the requirement of the NMDAR in activity-dependent synaptic plasticity, and the role the induction of synaptic plasticity has in regulating NSPCs and newborn neurons. We address how specific NMDAR agonists and antagonists modulate proliferation, both in vivo and in vitro, and then review the evidence supporting the hypothesis that NMDARs are present on NSPCs. We believe it is important to understand the mechanisms underlying the activation of adult neurogenesis, given the potential that endogenous stem cell populations have for repopulating the hippocampus with functional new neurons. In conditions such as age-related memory decline, neurodegeneration and psychiatric disease, mature neurons are lost or become defective; as such, stimulating adult neurogenesis may provide a therapeutic strategy to overcome these conditions. The adult brain continuously generates new neurons throughout life. These adult-born neurons arise from endogenous neural stem and precursor cells (NSPCs), which primarily reside within two neurogenic niches: the subgranular zone (SGZ) in the dentate gyrus (DG) of the hippocampus, and the subventricular zone (SVZ) of the lateral ventricles. In the 20 years since adult mammalian neurogenesis started receiving substantial attention [1,2], scientists have made great progress in understanding the mechanisms that regulate stem cell maintenance, proliferation, differentiation, maturation and integration [3,4]. Early on, extrinsic cues from the environment, such as learning [5], physical activity [6], and enriched environments [7,8], were found to potently enhance hippocampal neurogenesis. It was not long before a link was made to synaptic plasticity [9], which is also enhanced by physical exercise [10], and is widely accepted as the synaptic mechanism underlying hippocampal learning and memory [11]. Synaptic plasticity and adult hippocampal neurogenesis are intimately linked, as long-term potentiation (LTP), an activity-dependent change in synaptic efficacy, can influence the activation and proliferation of NSPCs in the DG (e.g., [12]), as well as the survival of newborn neurons (e.g., [13]). These newborn neurons have biophysical characteris-