Researchers have developed new tools to study microglia, the resident macrophages of the brain and spinal cord. These tools include monoclonal antibodies against the transmembrane protein Tmem119, which is specifically expressed by microglia in both mice and humans. The antibodies allow for the immunostaining and isolation of microglia, enabling detailed studies of their function in health and disease. The study also provides the first RNAseq profiles of highly pure mouse microglia during development and after immune challenges, revealing that microglia mature by the second postnatal week. These findings suggest that Tmem119 is a stable and specific marker for microglia, distinguishing them from other myeloid cells. The new tools will facilitate future research on microglial functions in the central nervous system. The study also highlights the importance of distinguishing microglia from other immune cells, as their unique developmental origins and functions are critical for understanding brain development and disease. The tools developed in this study, including the Tmem119 antibodies and RNAseq profiles, will be valuable for further research on microglial biology.Researchers have developed new tools to study microglia, the resident macrophages of the brain and spinal cord. These tools include monoclonal antibodies against the transmembrane protein Tmem119, which is specifically expressed by microglia in both mice and humans. The antibodies allow for the immunostaining and isolation of microglia, enabling detailed studies of their function in health and disease. The study also provides the first RNAseq profiles of highly pure mouse microglia during development and after immune challenges, revealing that microglia mature by the second postnatal week. These findings suggest that Tmem119 is a stable and specific marker for microglia, distinguishing them from other myeloid cells. The new tools will facilitate future research on microglial functions in the central nervous system. The study also highlights the importance of distinguishing microglia from other immune cells, as their unique developmental origins and functions are critical for understanding brain development and disease. The tools developed in this study, including the Tmem119 antibodies and RNAseq profiles, will be valuable for further research on microglial biology.