Experimental and human cancers are often characterized by the presence of tumor-associated glycosphingolipid (GSL) antigens defined by monoclonal antibodies. Major progress has been made during the past two decades on structural identification of these antigens. None of these structures are truly 'tumor-specific'. However, many of the antibodies show preferential or 'specific' reactivity with tumors, based on organizational differences of membrane GSLs in tumor cells versus normal cells. Clustered GSL antigens organized with transducer molecules in microdomain have been found recently to comprise a structural and functional unit involved in tumor cell adhesion coupled with signal transduction. Some of the GSL antigens have been identified as adhesion molecules recognized by carbohydrate-binding proteins or by complementary carbohydrates on target cells. Such adhesion, coupled with signaling, may initiate the metastatic process. Elucidating the mechanism of this initial adhesion/signaling step may lead to discovery of therapeutic agents that disrupt adhesion ('antiadhesion therapy') or normalize signaling ('ortho-signaling therapy'). Tumor-associated GSL antigens are also a target in immunotherapy of tumors, including development of antitumor vaccines.