Abstract: Microglia are brain resident immune cells with important functions across the lifespan.While in vivo microglial dynamics have been thoroughly characterized in the adult mouse brain, little is known about their normal dynamics during early development and aging.To gain more insight, we used in vivo two-photon imaging to explore cortical microglial dynamics in neonatal, adult, and aged microgliallabeled mice under normal physiological conditions and after laserinduced capillary injury.Under basal conditions, microglia in both neonates and aged mice have less ramified processes compared to the adult mice.Microglia in the neonate are higher in density, with more mobile somata and processes that extend/retract rapidly at baseline.While aged animals had similar microglia density as adults, their process were significantly less dynamic.During vascular injury in the adult brain, microglia cells extended their processes in a concerted fashion to surround and contain sites of injury.The aging brain is still capable of reacting to microvascular injury, though the amount of process extension declined.In neonates, the response was uncoordinated with some cells extending rapidly toward the injury, while other cells presenting delayed responses.Three days post-injury, microglial dynamics had largely recovered to baseline levels in the adult and aged brain, while neonates exhibited sustained microglial aggregation at the injury site and more broadly in surrounding brain regions.Our findings support the idea that basal and injury-evoked responses of microglial cells differ based on life stage.Our findings reflect microglial immaturity during development, and a decline of microglial function with aging.