Cell surface carbohydrates undergo dramatic changes in cancer. Changes in histo-blood group antigens result from either blocked synthesis with a precursor accumulation of oligosaccharide chain, or neosynthesis caused by the activation of 'silent' glycosyl transferases. Carbohydrate antigens play a crucial role in cell-cell communication, cell growth and differentiation. Although the biological significance of tumor-associated carbohydrate changes is not completely understood, these may provide the basis for uncontrolled cell growth, invasiveness and metastatic potential. Monoclonal antibodies developed against a variety of tumor cells have been identified as being directed against different carbohydrate antigens. Carbohydrate antigens and anti-carbohydrate antibodies are being used to develop diagnostic and therapeutic strategies for cancer.