Chondroitin sulfate proteoglycan 4 (CSPG4), also known as High Molecular Weight-Melanoma Associated Antigen, is a cell surface proteoglycan which has been recently shown to be expressed not only by melanoma cells, but also by various types of human carcinoma and sarcoma. Furthermore, at least in squamous cell carcinoma of head and neck and in basal breast carcinoma, CSPG4 is expressed by cancer stem cells. CSPG4 plays an important role in tumor cell growth and survival. These CSPG4-associated functional properties of tumor cells are inhibited by CSPG4-specific monoclonal antibodies (mAb) in vitro. Moreover, CSPG4-specific mAb can also inhibit tumor growth and metastasis in vivo. The anti-tumor effects of CSPG4-specific mAb are likely to reflect the blocking of important migratory, mitogenic and survival signaling pathways in tumor cells. These results indicate that CSPG4 is a promising new target to implement mAb-based immunotherapy of various types of cancer.