A diagnosis of mastitis is based on clinical observations or direct/indirect measures of the inflammatory response to infection, whereas a diagnosis of an intramammary infection is based on identification of the infectious agent. Somatic cell count/somatic cell score are common diagnostic tests for the detection of subclinical mastitis. Culture and polymerase chain reaction can be useful in the diagnosis of an intramammary infection; however, both have their advantages and disadvantages. Diagnosing the bacterial agent causing the intramammary infection can help to determine treatment and prevention strategies on the farm, which in turn can help to reduce incidence and prevalence.