Cancer is caused by genetic alterations. To understand the nature of these alterations–how they arise and how they are inherited–is to grasp the essence of cancer. In the past several years, hundreds of genes have been categorized as cancer genes. These genes, in turn, have illuminated basic pathways and regulatory networks that control cell fate. The identification of cancer genes and their respective functions in cells and tissues has revolutionized our view of tumors and how they grow. The primary literature that describes these intellectual strides is daunting in scope, but the central ideas can be readily condensed and simplified. The intent behind this book is to provide context to recent advances in cancer research by outlining basic principles that describe how cancer genes arise, are inherited, and function. Although the list of recognized cancer genes is likely to grow rapidly in the coming years, the fundamental principles of cancer genetics will likely endure.