Group I metabotropic glutamate receptors (mGlu1 and mGlu5) are coupled to polyphosphoinositide hydrolysis and are involved in activity-dependent forms of synaptic plasticity, both during development and in the adult life. Group I mGlu receptors can also regulate proliferation, differentiation, and survival of neural stem/progenitor cells, which further support their role in brain development. An exaggerated response to activation of mGlu5 receptors may underlie synaptic dysfunction in Fragile X syndrome, the most common inherited form of mental retardation. In addition, group I mGlu receptors are overexpressed in dysplastic neurons of focal cortical dysplasia and hemimegaloencephaly, which are disorders of cortical development associated with chronic epilepsy. Drugs that block the activity of group I mGlu receptors (in particular, mGlu5 receptors) are potentially helpful for the treatment of Fragile X syndrome and perhaps other neurodevelopmental disorders.