The ontogeny and transcriptional ‘signature’ of microglia are unique among tissue macrophages. However, reports of microglial plasticity have caused researchers to question the extent to which microglial identity is dictated by their yolk sac (YS) origins versus the environment that they experience within the CNS. Bennett et al. transplanted microglia isolated at different developmental stages, cultured microglia or macrophages derived from several other tissues into the brains of mice lacking endogenous microglia. All transplanted cell types rapidly engrafted in the parenchyma and exhibited microglial characteristics, suggesting that the CNS parenchymal environment provides signals that promote microglial identity; however, only those with YS origins fully recapitulated the microglial transcriptional signature, highlighting the importance of ontogeny for microglial identity.