Viral glycoproteins are found decorating mainly enveloped viruses and to a lesser extent non-enveloped ones. During virus morphogenesis the virion’s external proteins may undergo posttranslational modifications such as the addition of sugar moieties at specific amino acids, thus becoming glycosylated. Structural determination either by X-ray crystallography or transmission cryo-electron microscopy has classified these viral proteins based on their fold into classes I, II, and III. To mediate fusion with the host membrane and initiate the viral genome replication, all of them undergo substantial structural rearrangement transiting from a metastable pre- to a stable post-fusion conformation. Here, we revise the tertiary structure of viral glycoproteins, with an emphasis on the presence and importance of their decorating glycans. Deciphering the glycans’ presence, their structure and involvement in viral infection are not only critical for the development of antiviral treatments and vaccines but also a potential means to tune viral tropism and cell immune response by virus glyco-engineering.