Cancer, one of the defining personal anxieties of the twentieth century, is also one of the greatest biological puzzles that confront humanity. Perhaps a single cell, out of the trillions of cells in the body, starts a kind of rebellion against the great plan that gives each cell its identity. More genetic changes follow in the same cell or its descendants, to establish a dangerous malignancy. Biologists see the disease as a Darwinian struggle between the anarchic and outrageous stratagems of malignant cells and the organisational forces of the body. Probably only 20 percent of the risk of cancer is inherited; the remainder originates in life experiences that cause the genetic changes that start these insidious chain reactions. Like the ageing process considered in the last chapter, cancer originates in the absence of maintenance that is totally effective. Cancer is the most complicated and scientifically difficult disease we face, but our understanding is advancing sufficiently to allow hope for really effective control of ongoing cancers and the identification of environmental cues that initiate the disease.