Aging of the mammary gland is associated with numerous changes and is linked to higher cancer risk. To better understand these changes, the Brugge lab has performed single-cell transcriptomic analysis on a mouse model to generate a high-resolution atlas of the mammary glands at young and old age. The study, published in Cell Reports, identifies altered cell proportions and gene expression in several cell types, including lesser-known populations in the epithelial and immune compartments. The results highlight patterns of functional decline as well as a shift to a pro-tumorigenic and pro-inflammatory tissue microenvironment, which may contribute to the higher breast cancer risk associated with older age.