In his talk today, Dr. Hensch will discuss how maturing neural circuits are dramatically shaped by the environment, and how this timing varies across brain regions and plasticity declines with age. Focusing on cellular/molecular mechanisms underlying these developmental trajectories, they have identified specific events controlling the onset and closure of such 'critical periods'. The emergent balance of excitatory-inhibitory (E/I) circuitry triggers cortical rewiring. Targeting particular GABA circuits by pharmacological or genetic manipulations can either accelerate or delay critical period onset. Instead, plasticity appears to wind down when brake-like molecular factors emerge to stabilize adult networks. Lifting these brakes can reopen windows of circuit plasticity, carrying broad implications for understanding the etiology and potential therapeutic strategies toward neurodevelopmental disorders or recovery from brain injury in adulthood.