The process of lactation and feeding, referred to by some as the final stage of labour is remarkably successful. This phase of infant care has been subjected to considerable evolutionary pressure since the earliest mammals, reptiles and birds diversified. It has allowed thousands of species to occupy a vast range of ecological niches. Yet a significant complication of breast feeding remains inflammation of the lactating tissue: the pathology of mastitis. Mastitis rarely develops outside lactation, although it may affect individuals at any age in relation to congenital lesions such as duct ectasia, chronic disseminated infections such as tuberculosis, or during granulomatous, autoimmune or malignant processes. The immunology and consequences of mastitis as well as its impact on vertical transmission of infection require careful examination.