Monoclonal antibodies had the lure of drugs very much since their first description. The ability to bind to a predetermined chemical structure stimulated the imagination of drug discoverers and developers. Nevertheless it took many years before a drug was registered which started to make good on the promise. The complexity of the molecule, made up of four polypeptide chains, its large molecular weight, its multiple and versatile functional domains and its mouse origin initially were obstacles for the production and the utilisation. Also the selection of appropriate target structures on the surface of cells turned out be difficult. Many of these difficulties have been overcome. The replacement of most of the murine sequences with equivalent human sequences and the concomittant decrease in immunogenicity, and the identification of cell surface components which are causative and limiting in cellular transformation have made monoclonal antibodies valuable weapons in the fight against cancer. Multiple mechanisms of monoclonal antibody action are being exploited for this purpose. Antibodies can sequester growth factors and prevent the activation of crucial growth factor receptors. A monoclonal antibody directed against the vascular endothelial growth factor (VEGF) has been shown to be a potent neo-vascularisation inhibitor (bevacizumab). An antibody against the extracellular domain of the EGF receptor prevents the binding of the ligand to the receptor and thereby its activation (cetuximab). EGFR activity, however, is absolutely required for the survival and proliferation of certain human tumour cells. An antibody which interferes with the dimerisation of the ErbB2 and the ErbB3 members of the EGF receptor family prevents the association of a most potent signaling module (pertuxumab). The signals emenating from this dimer determine many phenotypic properties of e.g. human breast cancer cells. A monoclonal antibody also directed against ErbB2 has been most successful, clinically and commercially (trastuzumab). This antibody interferes with signals generated by the receptor and causes the arrest of the cell cycle in tumour cells. In addition, it recruits immune effector cells as cytotoxic agents. Finally, monoclonal antibody derivatives, single chain Fv fragments, have been used as a basis for the construction of recombinant tumour toxins. These molecules harness the exquisite binding specificity of the antibodies and combine them with the toxic principles of bacteria.