Early diagnosis is the most efficient way to decrease mortality rate, side effects and sequelae due to cancer. The first step consists in anticipating diagnosis by simultenaously taking into consideration non specific clinical symptoms and risk of cancer estimated by the combination of individual risk factors. Screening is a different way to anticipate diagnosis of cancer. Screening is a systematic investigation performed without clinical symptoms. Organised cancer screening programs allow quality control, evaluation and efficiency measure of the screening policy. Opportunist screenings as developed in some countries or for particular tumours are decided by practitioners and patients following practice recommendations. Side effects of these approaches have to be kept in mind: overdiagnosis and socio-economic inequalities must be controlled for both opportunistic and organised screenings.