There are disadvantages and benefits of population screening programmes. Although there is widespread public and political support for screening programmes of all kinds, important criteria must be applied before screening is introduced. The difference between population screening and opportunistic case-finding is identified here, and the costs, as well as the benefits, associated with screening are outlined. The importance of the difference between the sensitivity and the specificity of a test used in screening is explained. The economics of screening is considered, along with ethics and the importance of properly managing and evaluating screening systems. The current situation in the UK is described.