Lactation depends on milk secretion and the storage of the accumulating milk, followed by milk ejection and effective milk removal during milking. Environmental and disease factors may affect each of these stages independently and, thus, alter the milk composition. With normal milking intervals, the secretion of each of the milk constituents is continuous and linear with time. Subsequently, changes in milk composition occur during storage in the udder, mainly through the free movement of water between milk and blood and as a consequence of maintaining their osmotic equilibrium. The changes in composition are greatest in infected glands. The composition of milk varies in different parts of the udder: the fat content is much lower in the milk in the udder and teat sinuses. Following milk ejection, 10 to 20% of the milk is retained and cannot be removed by milking; the fat content of this residual milk will usually exceed 150 g/kg. The dynamics of these changes are complex and, whilst they may bring about large alterations in the composition of the milk obtained at individual milkings, their effect over longer periods is small. Thus, although incomplete milking, changing the milking intervals and increasing the frequency of milking will cause large fluctuations in the composition of milk at individual milkings, their longer-term effects are minor. The main effect of high levels of udder disease will be to reduce the lactose content but the serum protein level will increase and changes will occur in the concentrations of potassium, sodium and chloride ions. The design of milking equipment and the hygiene during farm storage of milk may materially alter the quality of milk for processing, because of lipolysis.