Rating scales of various sorts are very useful for both clinical and research purposes. However, they vary greatly regarding their reliability, validity, and utility. This article provides a guide for people who need to evaluate scales, either to incorporate them into their own research or clinical activities, or to determine if the results of studies which use scales are meaningful. The different types of reliability and validity are discussed and guidelines are offered to evaluate how well these were assessed. Finally, other aspects of scales which can affect their usefulness, such as completion time, training, and scoring ease, are discussed.