Recent and ongoing zoonotic infections of humans with avian influenza viruses have highlighted the importance of transmission in the development of an influenza pandemic. Despite the ability of H5N1 influenza viruses to grow to high titers and cause severe disease in human hosts, these viruses do not spread efficiently from human-to-human. The question of what viral, host and environmental factors are required to render an influenza virus transmissible has therefore become very topical. Recent work in the ferret model has suggested that receptor binding specificity is an important factor, but that the trait of human-like receptor recognition alone is not sufficient to confer a transmissible phenotype. In addition to the ferret, the guinea pig has been identified as a useful model host for transmission studies. Further research using these models is needed, toward understanding the molecular circumstances under which transmission can occur. A crucial role of antiviral drugs in mitigating an influenza pandemic will be to slow the spread of infection while an appropriate vaccine is in production. The efficacy of antivirals in preventing transmission is therefore of great importance. While the adamantanes, amantadine and rimantadine, have been found to fail in this respect due to the high transmissibility of drug resistant variants, the neuraminidase inhibitors, oseltamivir and zanamivir, show more promise. Anti-influenza drugs in development which show efficacy in terms of mitigating disease or viral growth should also be tested for their potential to block transmission.