The immune system, and in particular T cells, can be harnessed to treat cancer. Several bispecific T cell engaging antibodies of the BiTE^® format are in early or late-stage clinical development. These small recombinant antibody constructs effectively trigger killing of cancer cells by temporarily attached, polyclonal T cells. Blinatumomab, a CD19/CD3-bispecific BiTE^® antibody, has demonstrated high clinical activity in B cell leukemia and lymphoma patients. Three additional BiTE antibodies directed against surface target antigen expressed on solid tumors are being evaluated in phase I clinical trials. Alternative approaches to direct polyclonal T cells to kill cancer cells are under intense investigation.