Glycosylation is a very important posttranslational modification of many biologically relevant molecules. A change in the structure of glycans added to glycoproteins and glycolipids is a common feature of the change to malignancy. With the cloning of many of the glycosyltransferases and the identification of specific target molecules, it is now possible to define these changes at the molecular level and to dissect the mechanisms involved. Within the mammary gland, mucin-type O-linked glycosylation has been studied most extensively. In normal resting, pregnant and lactating breast, mucin O-glycans are largely extended (core 2 type) structures. In contrast, mucin O-glycans found in breast carcinomas are often truncated (core 1 type). One mechanism that is responsible for this increase in core 1 structures is a change in the expression of glycosyltransferases, particularly an increase in the expression of the sialyltransferase, ST3Gal-1. The loss, at least to some degree, of core 2 based glycans is a consistent feature of MUC1 mucin when it is expressed by mammary tumours as demonstrated by the unmasking of the SM3 epitope in greater than 90% of breast carcinomas.