Primary prevention of cancer requires control of both involuntary and voluntary exposures. Involuntary exposures include carcinogens in air and water, and various forms of radiation. Often these exposures are difficult to characterise individually and difficult to study epidemiologically. Although it is unlikely that they account for more than a small proportion of cancers, it is important that we refine our techniques of study to facilitate their control. Voluntary (lifestyle) exposures are responsible for the majority of cancers. In many developed countries, tobacco accounts for approximately 30% of cancer deaths, and major public health endeavours are justified to reduce this toll. Dietary factors may be as important, with dietary fat the most important risk factor, vegetables and fruits being protective. In several studies, including a cohort study in Canada, dietary fat increases breast cancer risk, though other studies have been negative. The evidence for fat increasing the risk of colorectal is more consistent. Epidemiology has shown that secondary prevention of cancer is applicable by screening for breast cancer with mammography with or without physical examination in women age 50-69, and screening for cervix cancer in women age 25-60 with cervical cytology. Organised screening programmes are essential to ensure that a high proportion of women are screened, and that the tests are high quality with adequate quality control. Under these circumstances screening every 2 years for breast cancer and every 3 years for cervix cancer is cost-effective. Screening for other cancers cannot be recommended currently. There is a time to effect that must be recognised in planning primary or secondary prevention. Full effect of most primary activities will not be achieved for decades, screening may require a decade. Available knowledge must be applied now, however, to ensure the effect will eventually be seen, as is now occurring in some countries with the downturn in lung cancer mortality following smoking reduction in men.