Specifying the delivery of a highly toxic agent to cancers while sparing normal tissue is an attractive prospect for the treatment of cancer. Cell surface antigens with highly restricted expression patterns have been identified and the means by which cytotoxic agents are appended to antibodies has been greatly refined. Myriad formulations involving radionuclides, bacterial toxins and small-molecule drugs linked to antibodies through peptides, protein fusions and chelating agents are in preclinical and clinical evaluation, and a few have been approved as human medicines. Recent advances hold promise for improving the therapeutic index of these anti-cancer drugs.