MUC1 mucins are highly glycosylated glycoproteins expressed on the luminal surfaces of glandular epithelia. In breast and ovarian carcinomas, their expression is frequently upregulated and they may be secreted into the circulation of cancer patients. Early studies aimed at the production of anti-MUC1 monoclonal antibodies revealed that MUC1 was a potent immunogen in mice with many monoclonal antibodies raised defining epitopes within the protein core of MUC1. The immunogenicity of MUC1 has now been extended to human studies and it is apparent that patients with breast and ovarian malignant disease are able to mount immune responses against MUC1. These findings provide information on the mechanisms involved in the recognition of MUC1 expressing tumours. The utilisation of MUC1 related immunogens to stimulate immune responses to tumours could lead to the improved management of patients and the development of new immunotherapeutic strategies aimed at the eradication of MUC1 mucin expressing cancers.