Sixty-five years ago the idea that the immune system serves as the body’s early warning system against cancer was proposed. But it took many decades of research to prove that “immune surveillance” was not only real but was critical for cancer prevention. In the last 15 years, scientists have learned how to harness and enhance parts of the cancer immune surveillance system. The result has been a revolution in cancer treatment in the form of an ever-increasing array of powerful immunotherapies.