All human cells show carbohydrate structures on the surface. New knowledge about the genetic mechanisms for the synthesis of these carbohydrates and the generation of monoclonal antibodies with high specificity shows that carbohydrates are involved in various cell-cell and cell-matrix interactions. For instance, cell surface carbohydrates seem to be important in connection with fertilization, embryonic development, cell differentiation, cancer, adhesion of microorganisms and immunological processes. This new knowledge will increase our understanding of various biological phenomena, and will thus be of value for diagnosis and treatment of various diseases.