The breast is the most prominent secondary sexual characteristic in women, and yet we know relatively little about how this tissue functions in health and disease. Our limited understanding of breast biology is exposed by the scant progress made towards preventing diseases such as breast cancer, and improving breast feeding rates. Our research investigates how the breast functions during key reproductive life stages including the menstrual cycle, lactation, and involution, and dissects the biology that underpins development of disease states. Our laboratory and others have demonstrated that macrophages perform a number of critical functions in the breast over the life course, they promote development of milk-secreting cells, phagocytose dying cells, participate in tissue remodelling and perform immune surveillance to protect against tumour formation. These diverse functions of macrophages are tightly regulated within the mammary microenvironment by hormones, innate immune receptor signalling, cytokines, and chemokines, and inappropriate macrophage activation can lead to inflammation, lactation insufficiency and breast cancer. This research is uncovering novel treatment and prevention strategies for breast disease which may in the future lead to improved breast health for women.