Microglia form the first line of defence for the neural parenchyma. But do these cells pursue an active role in the normal brain, or do they become activated only after injury? Two papers published recently by Nimmerjahn et al. and Davalos et al. used in vivo two-photon laser-scanning microscopy reveal that the fine branches of 'resting' microglia are highly mobile, and provide extensive and continuous surveillance of their cellular environment. These moving branches show a rapid chemotactic response to tissue injury that depends on purine receptors and connexin hemichannels, and they appear to take cues from surrounding astrocytes both in the normal and the injured brain.