YKL-40 is a recently discovered human glycoprotein which is related in amino acid sequence to the chitinase protein family, but has no chitinase activity. The present review focuses on the expression and regulation of YKL-40, its clinical significance, detection techniques and outlines the advantages and limitations of its application as a novel biomarker. YKL-40 is expressed and secreted by macrophages, neutrophils, fibroblast-like synovial cells, chondrocytes, vascular smooth muscle cells and hepatic stellate cells. The complete biological function of YKL-40 is unclear, and it is not yet known to have a specific receptor. However, its pattern of expression is associated with pathogenic processes related to inflammation, extracellular tissue remodeling, fibrosis and solid carcinomas. It is assumed that YKL-40 plays a role in cancer cell proliferation, survival, invasiveness and in the regulation of cell-matrix interactions. It is suggested that YKL-40 is a marker associated with a poorer clinical outcome in genetically defined subgroups of different tumors. YKL-40 was recently introduced into clinical practice and its application is still restricted. There are a few techniques available for its detection and the commercially accessible kits are limited, too. Elucidation of YKL-40 functions is an important objective of future studies as it seems likely that YKL-40 might have a significant role in cancer invasiveness and could possibly serve as an attractive target in anticancer therapy.