This article focuses on how to do meaningful power calculations and sample-size determination for common study designs. There are 3 important guiding principles. First, certain types of retrospective power calculations should be avoided, because they add no new information to an analysis. Second, effect size should be specified on the actual scale of measurement, not on a standardized scale. Third, rarely can a definitive study be done without first doing a pilot study. Some simple examples as well as a complex example are given. Power calculations are illustrated using Java applets developed by the author.