The purpose of cancer screening tests is to detect pre-cancer or early-stage cancer in asymptomatic individuals so that timely diagnosis and early treatment can be offered, where this treatment can lead to better outcomes for some people. The aim of a cancer screening programme is either to reduce mortality and morbidity in a population by early detection and early treatment of a cancer (for example, breast screening) or to reduce the incidence of a cancer by identifying and treating its precursors (such as cervical and colorectal screening). This short guide is designed to be a quick reference that contains the important ideas about cancer screening. Readers should refer to other publications for comprehensive discussion and detailed guidance on cancer screening programmes.