Drug development in oncology has traditionally focused on therapies that directly target cancer cells—surgery, radiation therapy, chemotherapy, and kinase inhibitors. However, for more than a century it has also been recognized that the immune system interacts with cancer. The immune surveillance hypothesis, proposed in the 1950s, posited that the immune system could recognize and reject cancer cells as being foreign, in the same way that it reacts against microbial agents and transplanted organs. Initial attempts to harness the potential of cancer immunotherapy by directly activating antitumor immunity with cancer vaccines or recombinant cytokines, or by infusing tumor-specific immune cells prepared in the laboratory, had limited success. It was not until scientific research provided a greater understanding of interactions between cancer and the immune system that a quantum leap in immuno-oncology occurred.