Mastitis is the most prevalent bacterial disease among dairy cows, and the majority of antimicrobials given to dairy cows are for the prevention or treatment of this disease. Thus, a thorough understanding of mastitis is necessary when considering the appropriate use of antibiotics. After reviewing how antimicrobials are currently used on dairy farms, this chapter considers recent research on the prevalence and development of antimicrobial resistance in mastitis pathogens. It then shows how consistent diagnostic protocols and recording systems, attention to medical history, appropriate choice of antibiotics and control of treatment duration can all contribute to minimizing unnecessary use of antimicrobials and promoting effective treatment of mastitis.