Influenza viruses attach to cells by binding to glycans, especially those terminating with sialic acid, through hemagglutinin present on the particle surface. Sialosides as influenza virus receptors have been broadly divided into two categories based on their structure, Siaα2–3Gal (avian-type) and Siaα2–6Gal (human-type) [1]. In recent years, with the development of glycan array technology, it has been found that the binding of the virus and the glycan is affected not only by the terminal α2–3/α2–6 linkage of Sia and Gal but also by the number of branches and LacNAc repeats [2], sulfation [3], fucosylation [4], and the molecular species of sialic acid [5]. The amino acids on the hemagglutinin determining the binding specificity toward sulfated and fucosylated receptors have been identified, while there are still many unknown points about the mechanism by which hemagglutinin distinguishes the number of branches and LacNAc repeats. Furthermore, little is known about the detailed structure and diversity of sialic acid glycans in the tissues of host. With the spread of high pathogenicity avian influenza viruses worldwide in the 2020s, virus infections have been recognized in various wild birds, carnivorous or marine mammals, and even in animals such as dairy cows that had not been considered as susceptible host before. It is desired to evaluate the risk of infection to various animals, not only humans and livestock, while monitoring changes in the receptor specificity of the virus (Fig. 65.1).