MUC1 is a large surface glycoprotein expressed by epithelial cells, which is over expressed and aberrantly glycosylated in adenocarcinomas and in haematological cancers. MUC1 is a target antigen and has been widely targeted for tumour immunotherapy studies in mice and in human clinical trials. Recent evidence has suggested that MUC1 has immunosuppressive properties, which may promote cancer progression and metastasis. In addition, MUC1 has been identified in the reproductive tract, on immune cells (such as T cells, B cells, dendritic cells, monocytes), to have anti-adhesive properties and protect against infections. However, no precise function has been ascribed to this molecule. Herein, we will review MUC1 in cancer, in disease, on immune cells, suggested functions, interaction with pathogens and evidence in cell signalling.