Concern about antimicrobial usage (AMU) in food animals and development of antimicrobial resistant microorganisms continues to grow and regulators are increasing supervision of AMU on dairy farms. The recently enacted Veterinary Feed Directive is an example of increased restrictions on previously OTC drugs and increased veterinary oversight of antimicrobials given in feed and water. On dairy farms, antimicrobials are used to treat bacterial diseases and treatments for mastitis, lameness, respiratory diseases and reproductive and digestive disorders are the most common reasons dairy cows receive antimicrobials. Studies conducted in North America and Europe have quantified AMU using standardized indices (Defined Daily Doses). While calculations vary, most researchers have reported that dairy cows receive about 5 – 8 DDD of antibiotics per cow per year and about 35–85% of the doses are given via intramammary (IMM) infusion, with the remainder given parenterally or orally (to calves). In most studies, mastitis is the most common reason for AMU. Increased regulations on AMU are based on assumptions that reduced usage will result in decreased selection for resistant organisms and reduced threats to human health. However, on dairy farms, AMU has been relatively restricted and evolution and maintenance of resistant pathogens in dairy cows or farm environments has not been well described. Increased resistance of fecal E. coli has been reported after systemic administration of ceftiofur, but susceptible bacterial populations rapidly rebounded after treatment ended. Similar trends have been noted in farm environments. Some researchers have demonstrated variation among farms and organisms in resistance to some drugs and some associations of resistance with AMU has been noted but these associations have been limited to pathogens recovered from milk. While AMU for mastitis consumes the greatest quantity of antimicrobials, several reviews have stated that there is relatively little evidence to suggest that widespread resistance or lack of efficacy is emerging or progressing. However, more research on how to improve AMU on dairy farms is needed and mechanisms to increase involvement of veterinarians in treatment decisions are warranted.