Cancer is the second leading cause of death in the world. Almost everyone’s life is in some way or other affected by cancer. Yet, when faced with a cancer diagnosis, many of us will confront questions we had never before considered: Is cancer one disease, or many? If many, how many exactly? How is cancer classified? What does it mean, exactly, to say that cancer is “genetic,” or “familial”? What exactly are the causes of cancer, and how do scientists come to know about them? When do we have good reason to believe that this or that is a risk factor for cancer? These questions are (in part) empirical ones; however, they are also (in part) philosophical. That is, they are questions about what and how we come to know. They are about how we define and classify disease, what counts as a “natural” classification, what it means to have good evidence, and how we pick out causes as more or less significant. This book takes a close look at these philosophical questions, by examining the conceptual and methodological challenges that arise in cancer research, in disciplines as diverse as cell and molecular biology, epidemiology, clinical medicine, and evolutionary biology.