It’s no secret that chemotherapy is a brutal and all-too-often unsuccessful way to treat cancer, one that harms healthy cells even as it tries to ferret out malignant ones. The search for alternatives is leading researchers deep into the human immune system, seeking ways to exploit the body’s extraordinary drive to heal. They are going to the frontlines of science to understand the pathways that regulate antitumor immunity, using nanotechnology to design tumor-inhibiting immunotherapies, and building models that use immune cells as drug-delivery devices. What is the role of the immune system in the progression of cancer? How can it be enlisted to halt tumor growth?