Glycosylation is an important post-translational modification of proteins, lipids and other biomolecules, essential in a plethora of physiopathological processes. It plays a significant role in protein folding, stability, trafficking, and recognition events, impacting cellular communication, immune response, development, and disease progression. The great diversity and heterogeneity of glycans structures, assembled and remodeled by a complex machinery of glycosyltransferases and glycosidases, indeed, encodes several information that is context-dependent due to the cell-specific patterns of glycosylation. In this chapter, we will discuss the protein glycosylation pathway in eukaryotes, procaryotes and viruses. Examples of glycoproteins functions in different species are also reported to highlight the crucial roles of glycans in diverse biological processes in almost all living organisms.