Summary Influenza virus is an enveloped virus with 8 single stranded RNA fragments as genome. Viral RNA polymerase poor fidelity induces a high genetic variability for this pathogenic agent. Two different mechanisms account for this phenomenon. Antigenic drift, which is the most frequent, corresponds to punctual mutations within the genome. Antigenic shift, which is more drastic, occurs through genetic recombination, essentially within the pork, between viruses from different origins (swine, human or avian) leading to the emergence of new variants. Those new viruses are responsible for global pandemics like the massive-killing 1918's flu or the more recent 2009-2010 event. Existing antiviral treatments consist in two classes of drugs namely, adamantanes and neuraminidase inhibitors. Adamantanes have been extensively used during the last decades and have led to the emergence and selection of naturally drug-resistant strains. Currently, new resistant viruses to neuraminidase inhibitors have been described, probably due to the massive prescription of these drugs, both for prophylaxis and curative treatments. Phenotypic and genotypic assays, based on IC50 determination and molecular biology techniques (Real time PCR or sequencing) respectively, can be used for the identification and characterization of those resistance mutation carrying viruses.