Introduction of new animals to dairy herds has become increasingly common, both as a method of replacing culled animals and to increase the number of cattle during herd expansions. Biosecurity for control of mammary disease refers to the efforts taken to a) reduce the introduction of mammary pathogens to a naive herd via introduced cattle, and b) reduce the spread of these pathogens between members of the same herd. Biosecurity has been recognized as an important means of control of mammary disease long before the term came into common usage. Veterinarians can help producers avoid substantial losses by insisting that they implement biosecurity measures for control of mastitis. This presentation will discuss the major mastitis pathogens with particular emphasis on their origins and control measures, and then will describe tests used for screening cattle for the presence of contagious mastitis pathogens.