Glycans are carbohydrate structures coating the cell surface of virtually all nucleated cells of vertebrates. A crucial function of glycans is in cell interaction and recognition, which is constantly exploited by bacteria and viral pathogens as targets for toxins, and as a receptor to aid recognition of and adhesion to host cells. Sialic acids, a 9-carbon compound commonly added as a terminal sugar on glycoconjugates is crucial to both bacteria and viruses causing a successful infection. Bacterial pathogens utilize sialic acids as a component of LOS and capsule, both key virulence factors. The decoration of the bacterial cell surface with sialic acid glycoconjugates has two roles; first, to mimic host glycans to evade the host immune system, second, in reducing the efficiency of complement mediated lysis and opsonophagocytosis. Several key bacterial pathogens including Haemophilus influenzae and Pneumococcus also utilize sialic acids as a carbon source. In viruses, host sialic acids glycoconjugates act as a receptor for viral recognition and adherence to host cells. In the case of influenza A virus, a viral neuraminidase is required to cleave and release newly formed viral particles from infected host cell. This thesis investigates the role of sialic acid glycoconjugates in pathogenesis to increase the understanding of sialic acid biology in human host adapted pathogens. The role sialic acids play in pathogenicity is investigated in the major human pathogens; non-typeable Haemophilus influenzae (NTHi) and Neisseria meningitidis and in human influenza A viral strains.