Assessing the quality of randomized controlled trials is a relatively new and important development. Three approaches have been developed: component, checklist, and scale assessment. Component approaches evaluate selected aspects of trials, such as masking. Checklists and scales involve lists of items thought to be integral to study quality. Scales, unlike the other methods, provide a summary numeric score of quality, which can be formally incorporated into a systematic review. Most scales to date have not been developed with sufficient rigor, however. Empirical evidence indicates that differences in scale development can lead to important differences in quality assessment. Several methods for including quality scores in systematic reviews have been proposed, but since little empirical evidence supports any given method, results must be interpreted cautiously. Future efforts may be best focused on gathering more empirical evidence to identify trial characteristics directly related to bias in the estimates of intervention effects and on improving the way in which trials are reported.