Structure-based design and synthesis of potent influenza virus neuraminidase inhibitors are now being evaluated in human trials as anti-influenza virus drugs. The first drug of this class, Relenzaβ’ (Zanamivir/GG167), is now awaiting pharmaceutical evaluation and registration in Australia, Europe, and North America for both treatment and prophylaxis of influenza. The target for the drug is the active site of neuraminidase, which is a pocket that has been totally conserved in both Type A and B influenza in all known subtypes of influenza (animal and human). Mutations in residues that surround this conserved pocket allow the virus to escape binding to circulating antibodies that recognise the molecular surface around the active site of the wild-type virus. High-affinity neuraminidase inhibitors have been designed that interact only with the conserved active site residues. The design of these sialic acid analogues was based on the crystal structure of influenza virus neuraminidase and its complex with N-acetyl neuraminic acid (sialic acid) and 2-deoxy-2,3-dehydro-N-acetyl neuraminic acid. These novel inhibitors are highly specific for influenza neuraminidase, and have been shown to inhibit influenza virus replication in both cell culture and animal models. The development of drugs against a rapidly mutating organism like influenza has to address to the possibility of emerging drug resistance. This is examined in the light of drug resistant mutants selected after in vitro passaging of virus in the presence of neuraminidase inhibitors.