A very wide range of homeostatic processes are disturbed in cancer. These are visible at patient, lesion, tissue and cellular levels and are increasingly understood at a molecular level, providing many opportunities to research molecular therapeutic targets. This chapter introduces and contextualises most of the processes currently understood as of importance to the biology of cancer: many of these understandings are comparatively new; some are speculative. It is upon such speculations and the application of scientific method that progress in cancer prevention and management depend.