T cell receptors are antigen-specific proteins that have evolved to recognize peptide antigens presented by human leukocyte antigen molecules on most cell types. Like antibodies, T cell receptors are produced with huge diversity but, unlike antibodies, T cell receptors are not secreted and do not undergo somatic mutations that increase their affinities for antigen. Recently, however, methods have been developed that enable T cell receptors to be engineered as soluble proteins with extremely high affinities, and fused to various immune-modulator molecules. T cell receptors are now set to unlock a whole new range of targets with key roles in cancer, viral infections, autoimmune diseases and allergies.