An ideal molecule to deliver radioimmunotherapy (RIT) would be target specific and have prolonged residence time at high concentrations in the tumour with rapid clearance from normal tissues. It would also be non-immunogenic. These features can be rationally introduced into recombinant antibody-based proteins using antibody engineering techniques. This review focuses on the use of antibody engineering in the design and development of RIT molecules which have single chain Fv (scFv) antibody fragments as building blocks.