The influenza virus A hemagglutinin (HA) is a trimeric glycoprotein that contains 3-9 N-linked glycosylation sequons per subunit, depending on the strain. The location of these sites is determined by the nucleotide sequence of the HA gene, and, since the viral genome is replicated by an error-prone RNA polymerase, mutations, which add or remove glycosylation sites, occur at a high frequency. Mutations that are not lethal to the virus add to the structural diversity of the virus population. Factors that determine the glycosylation of the HA are reviewed herein, as are the effects of host-specific glycosylation on receptor binding, fusion activity, and antigenic properties of the virus. Effects of host-specific glycosylation and selection on virulence and on vaccine efficacy and surveillance are discussed. In addition, inadequacies in our understanding of HA glycosylation and its effects on host range are emphasized.