Mannheimia haemolytica is a leading contributor to bovine respiratory disease (BRD). Various vaccines are available for prevention or control of BRD due to M. haemolytica; published research indicates that vaccines can decrease BRD morbidity. Several antimicrobials are labeled for treatment or control of BRD due to M. haemolytica. Historically, metaphylaxis (mass antimicrobial medication at arrival) has been shown to reliably decrease BRD morbidity in highrisk cattle. Until recently, antimicrobial resistance (AMR) in M. haemolytica has been rare, even in cattle given antimicrobials. However, in the past 5 years multiple reports described M. haemolytica isolates from fatal BRD cases that are resistant to many or most antimicrobials. Additionally, nasopharyngeal shedding of AMR M. haemolytica by large proportions of live high-risk cattle following metaphylaxis has been reported. This information suggests that response rates following antimicrobial administration for BRD treatment or control may begin to decline. However, to date, reported death rates have not been high in cattle described to be shedding AMR M. haemolytica, and retrospective studies have not revealed a direct relationship between prevalence of AMR M. haemolytica and treatment failure. Properly designed prospective studies are needed to more clearly define the impact of AMR M. haemolytica in cattle populations.