Please cite this paper as: Govorkova EA. (2012) Consequences of resistance: in vitro fitness, in vivo infectivity, and transmissibility of oseltamivir-resistant influenza A viruses. Influenza and Other Respiratory Viruses 7(Suppl. 1), 50-57. The development of drug resistance is a major drawback to any antiviral therapy, and the specific anti-influenza drugs, the neuraminidase (NA) inhibitors (NAIs), are not excluded from this rule. The impact of drug resistance depends on the degree of reduction in fitness of the particular drug-resistant virus. If the resistance mutations lead to only a modest biological fitness cost and the virus remains highly transmissible, the effectiveness of antiviral use is likely to be reduced. This review focuses on the fitness of oseltamivir-resistant seasonal H1N1 and H3N2, 2009 pandemic H1N1 (H1N1pdm09), and highly pathogenic H5N1 influenza A viruses carrying clinically derived NAI resistance-associated NA mutations.