Alterations in glycosylation are hallmarks of malignant transformation in the gastric and colorectal tract. Altered protein glycosylation favors malignant phenotypes and contributes to poor outcome. Advances in glycomics, glycoproteomics and glycoengineering have broadened our understanding of the human glycoproteome. A comprehensive glycomic and glycoproteomic characterization of human tumors will allow true precision medicine setting. Cancer-associated glycans hold tremendous potential for non-invasive cancer detection and targeted therapeutics.