YKL-40, a member of ‘mammalian chitinase-like proteins’, is secreted by macrophages, neutrophils, chondrocytes, endothelial-, vascular smooth muscle-, and cancer cells. High serum YKL-40 is a biomar-ker of poor prognosis in patients with cancer, inflammation and increased tissue remodelling. High YKL-40 protein expression assessed by immunohistochemistry is found in breast carcinomas associated with short disease-free survival and in glioblastomas with increased resistance to radiotherapy and decreased overall survival. In this chapter we describe the methods for the detection of (1 ) YKL-40 protein expression in human tissues (using immunohistochemistry) and cell cultures (using immunocytochemistry); (2 ) YKL-40 mRNA expression in human tissues (using in situ hybridization and Polymerase Chain Reaction); and (3 ) YKL-40 protein concentrations in serum or plasma (using Enzyme-Linked ImmunoSorbent Assay).