Mastitis is a significant problem amongst lactating women yet there remains a paucity of scientific research into the anatomical, physiological and pathological determinants for mastitis. There is also scant knowledge regarding the physiological changes occurring within the breast as a result of mastitis. This paper examines the available research and current clinical and scientific opinion concerning the breast's response to inflammation and infection and the numerous influences that may impact upon the development of mastitis. In particular, the difficulties associated with differentiating between infective and non-infective mastitis are discussed.