Influenza virus infections in high risk individuals, such as infants, the elderly, and patients with cardiopulmonary disorders or immunocompromised states, cause severe manifestations which often result in fatalities. The emergence of a new antigen type of influenza A virus (H5N1) in Hong Kong during 1997 and 1998 threatened a possible pandemic of a new influenza infection.The investigation for anti-influenza chemotherapies has progressed in the last decade whereas clinical trials of new compounds have been limited to amantadine, rimantadine and ribavirin. Fusion inhibitors which directly inhibit conformational change of haemagglutinin (HA), protease inhibitors which inhibit cleavage of HA to HA1 and HA2, RNA transcription inhibitors which inhibit cap formation of mRNA and antisense oligonucleotides targeted at mRNA of PB2 (a part of viral RNA polymerase) have been reported, in their development phases.Recently, 2 neuraminidase (NA) inhibitors, zanamivir and oseltamivir (GS 4104), were used in clinical trials for the treatment of patients with influenza. Both agents showed promising results. A polyoxometalate, PM-523, inhibits fusion between the virus envelope and cell membrane and inhibits the penetration of the virus into cells. This compound has shown potent anti-influenza activity and synergistic inhibitory activity in combination with ribavirin or zanamivirin vitroandin vivo.Resistant strains for zanamivir, oseltamivir or PM-523 have been isolated. The analysis of mutation points of these strains have contributed to the investigation of the antiviral mechanisms of action of these compounds and the mechanism of resistance of the mutants to these compounds.