The selective recognition of tumour cells by monoclonal antibodies, labelled with radioactive isotopes, for use in diagnosis and treatment, forms the basis of immunoscintigraphy, radio-immunoguided surgery and radio-immunotherapy. Research into the application of these systems has encountered multiple difficulties, most notably a low tumour to non-tumour ratio of radioactivity. The development of pretargeting systems, separating the individual steps of tumour cell targeting and the introduction of the radioactive label, have led to significant increments in tumour to nontumour ratios and an improvement in diagnostic accuracy. Before pretargeting strategies are applied clinically, a thorough understanding of these systems is required and forms the backbone of this report. Clinical examples of early trials have already confirmed many of the theoretical advantages of pretargeting systems and new protocols are already being investigated.