Recent progress in the studies on the roles of carbohydrates has brought about critical discoveries, which allow us to have working hypotheses for understanding the roles of tumor-associated carbohydrate antigens. In this report, I focus my description on three different aspects of this progress. I discuss: (a) the immunological response to oligosaccharides aberrantly expressed under pathological conditions; (b) possible roles of carbohydrate-dependent cell adhesion during tumor metastasis; and (c) the roles of carbohydrates in modulating the functions of proteins that attach those carbohydrates. These three areas in the roles of carbohydrates will likely be the target for continuous research efforts to reveal the roles of tumor-associated carbohydrates.