Existing scientific trials, despite their flaws, support mammography screening programs. In addition, there are good reasons to believe that modern, wellconducted screening programs may achieve earlier detection and diagnosis of breast cancer and, perhaps, greater reductions in breast cancer mortality than what has been found in screening trials. Trial data published to date do not provide scientific justification to recommend screening for women younger than 50. However, this conclusion does not exclude the possibility that screening of individual women, based on a personalized risk assessment, could be of benefit. These conclusions should be reviewed when results from the UK Trial become available. Modern screening programs such as the PQDCS may produce outcomes comparable or even superior to those observed in screening trials if they achieve a standard of quality equal to or better than the standard achieved by trials. Measures that should reduce false positive rates and assure high quality screening include making sure that high quality mammographic films are being produced, that readers have the necessary expertise to detect early cancer and avoid false positives, and double reading of a proportion of films. While participation rates should be as high as possible, efforts to increase participation should not overstate the benefits of mammography nor understate the risks and uncertainties which remain.