Monoclonal antibodies have become the most effective therapeutic modality for treating human cancer. Anti-cancer monoclonal antibodies can be targeted against cancer cells by several mechanisms. Even unconjugated antibodies show significant efficacy in the treatment of solid tumors and hematological malignancies. Immunoconjugates composed of antibodies conjugated to chemo-drugs, radioactive or toxins are powerful therapy for lymphomas and solid tumors. Moreover, immunomodulatory antibodies can promote the induction of anti-tumor immune responses by directly activating or inhibiting molecules of the immune system. Antibody structures now can be readily manipulated to facilitate selective interaction between the immune system and cancer cells, so that these reagents will become important components of the anti-neoplastic protocols of the future.