Macrophages are a common cell type in every tissue in the body, but their abundance and location was not fully appreciated until the advent of lineage-specific monoclonal antibodies in the 1980s. The images in this section were produced by immunohistochemical localisation of the F4/80 antigen in mouse tissues. F4/80 (which is identical to BM8) antigen is present on an integral membrane protein called emr1, which is a G protein-coupled receptor of unknown function that has extracellular EGF and mucin-like repeats. The high resolution of these images, which mainly represent immunoperoxidase staining of perfusion-fixed and paraffin-embedded tissues, highlight the extensive networks of macrophage membrane process and the intimate connections between resident tissue macrophages and other cell types. Each image is annotated with information about the location and possible function of tisssue macrophages. F4/80 does not label all macrophages. An alternative view is obtained from our MacGreen mice, in which a transgene tags all the macrophages through expression of green fluorescent protein (see Transgenic Animals).