The article describes the most important role of carbohydrates in post-translational modification of proteins in plant and animal cells. It is noted that it is protein modification with carbohydrates that is the reason for proteins' diverse functional capabilities. The processes of glycan synthesis on the endoplasmic reticulum and their attachment to proteins, followed by transport to the Golgi apparatus, are described. The subsequent fate and function of the resulting glucoconjugate in the cell are discussed. The article addresses the necessity of further glycan research for modifying cellular processes and consequently creating plant and animal organisms equipped with new properties, and examines contemporary spectral and chromatographic methods for glycan research.