Mucins are high molecular weight glycoproteins with complex oligosaccharide side-chains attached to the apomucin protein backbone by O-glycosidic linkage, found both in crude mucus gels that protect epithelial surfaces in the major tracts of the body and as trans- membrane proteins expressed on the apical cell surface of glandular and ductal epithelia of various organs. Changes in the sequence of glycosylation of mucins in different settings generate a variety of epitopes in the oligosaccharide side-chains of mucins, including newly expressed blood-group antigens, distinguishing between normal and diseased states. Tumour-associated epitopes on mucins and their antigenicity make them suitable as immuno-targets on malignant epithelial cells and their secretions, creating a surge of interest in mucins as diagnostic and prognostic markers for various diseases, and even influencing the design of mucin-based vaccines. In this review, I discuss the emerging roles of mucins such as MUC1 and MUC4 in cancer and some other diseases and stress how under-glycosylated and truncated mucins are exploited, both as markers of disease and to monitor widespread metastasis, making them useful in patient management. Furthermore the type, pattern and amount of mucin secreted in some tissues have been considered in the classification and terminology of neoplasia and in specific organs such as the pancreas. These factors have been instrumental in pathologic classification, diagnosis and prognostication of neoplasia.