We begin by looking at cancer patterns worldwide. These are of interest because they show marked variations in the forms of the disease that afflict different populations. These differences indicate the importance of environmental factors that include lifestyle – for example, what we eat and tobacco use – in determining both the type of cancer and the frequency of occurrence. Although there is variation in cancer types, there is a broad trend of rising incidence across the world, for which a major driving force is increasing longevity. In the developed world lung, breast, bowel and prostate cancers head the mortality table. Taking all cancers together, the last 30 years has seen a gradual increase in the five-year survival rate, although there remain significant variations between nations and even within some countries. For the developing world the outlook is more depressing: not only is the annual number of new cases rising but inadequate screening programmes often mean that diagnosis is delayed until tumours have spread to secondary sites in the body and therefore become very difficult to treat. Analysis of cancer mortality in different age groups revealed many years ago that the additive effects of about half a dozen discrete events drives cancer development – the first direct evidence that the accumulation of mutations is the underlying cause.