United Kingdom Lung Screening trial is a large national research project which we hope will benefit people at risk of developing lung cancer. Anybody can develop lung cancer, but people who smoke (or used to smoke) are at the greatest risk. The risk increases with the total number of cigarettes you smoke. If you stop smoking, the risk gets less over time. Passive smoking (breathing in someone else's tobacco smoke) over a long period of time may also cause lung cancer. Non smokers may develop a particular type of lung cancer (called adenocarcinoma). This can happen in a part of the lung where damage has happened for a number of other reasons (such as breathing in asbestos). Lung cancer screening uses an advanced x-ray technique, called computed tomography (CT) scanning, in order to find lung cancer before symptoms develop so that early treatment can remove the cancer and deaths from lung cancer may be prevented. More people die from lung cancer in the UK every year than from any other form of cancer. About 40,000 people develop lung cancer each year in the UK and many of these are smokers. However, non-smokers and ex-smokers can also develop lung cancer. Finding lung cancer by screening instead of symptoms may mean that treatment is more effective. Lung cancer screening aims to find changes in the lung at the time of the scan but it cannot prevent you from getting lung cancer in the future. Only one randomised controlled screening trial (NLST) so far has shown that screening by CT scanning is effective. The NLST trial has been recently published and was conducted in the USA. The results showed that the mortality from lung cancer was reduced by 20%. Thus it is likely that in the same population, CT screening is effective. However, populations differ and there are other questions that remain unanswered, regarding the integration of CT screening into a national programme and which patients benefit most. The unanswered questions led a group of experts from many countries to conclude that further trials (including UKLS) were needed to answer these important questions and to guide the design of future screening programmes.