The teat canal of lactating dairy cattle is of particular importance for the defence of facultative pathogenic and pathogenic microorganisms (e. g. staphylococci) invading the bovine udder. Furthermore the canal is usually colonized with microorganisms, too. In addition to microorganisms inhibiting mastitis pathogens the teat canal is colonized by staphylococci. The microbial colonization can be influenced by the environment of the animals, the care and disinfection of the teat skin and indirectly by the effects of forces being associated with machine milking. Because of vacuum fluctuations occurring under the teat tip microorganisms, which colonize the teat canal, can invade the bovine mammary gland and cause infections there. This paper gives a review of the microbial colonization of the bovine teat duct and of influencing factors on the microbial populations as well as of the significance of the teat canal colonization for the development of mastitis.