Background: Glycosylation is a posttranslational modification that is involved in many biological processes and significantly affects the processes associated with tumour progression. Changes in glycan structures on the surface of tumour cells caused by altering levels of glycosyltransferase and glycosidase expression affect proliferation, adhesion, migration and cellular signalling. The presence of aberrant glycan structures and glycoconjugates in the sera of oncological patients has been reported in many cancers. Consequently, many glycoproteins have been approved by the U.S. Food and Drug Administration as tumour biomarkers for clinical investigations. At present, attention is focused on the search for new glycomarkers that are decorated by aberrant glycosylation or are overexpressed in the serum or exosomes due to their active secretion or release from tumour cells to the extracellular space. Purpose: The aim of this article has been to review the structure of glycans, glycoproteins and other glycoconjugates and to give more details about their functions in the development and progression of tumours. Another aim was to familiarise the reader with selected clinically approved glycoproteins used to diagnose oncological diseases (AFP, PSA, CA 125, HE4). Attention was paid to changes in the glycan structure of these proteins, their function, serum concentrations and clinical use in the diagnostics of cancer.