Cows are milked at least twice a day on farms worldwide. Three main sources of microbial contamination of raw milk can be distinguished: the interior of the udder, the exterior of the teats and the udder and milking and storage equipment. These main sources are discussed, based on literature. The microbiological infection of milk taking place inside the udder varies considerably, even when cows are healthy. The main groups of micro-organisms are micrococci (including staphylococci) and streptococci. In case of mastitis, counts of streptococci, staphylococci or coliforms will be as high as the total plate count. The exterior of the udder can be an important source of contamination. In general the teat contamination of housed cows is higher than the teat contamination from cows at pasture. The groups of micro-organisms isolated from teats are mainly micrococci and aerobic sporeformers. Before milking, udder preparation takes place in many cases. A tenfold reduction of teat contamination with spores can be obtained with special preparation methods. Most of the pathogenes in raw milk originate from the interior and the exterior of the udder. During milking, the major source of bacteria in milk is the milk contact surfaces of milking equipment and milk cans or bulk tanks. High numbers of bacteria can be found. After production, milk is stored in a bulk tank before collection. During storage and transport, the microflora in milk changes; the psychrotrophic counts increase.