ABSTRACT—There is no universal guideline or rule of thumb for judging the practical importance or substantive significance of a standardized effect size estimate for an intervention. Instead, one must develop empirical benchmarks of comparison that reflect the nature of the intervention being evaluated, its target population, and the outcome measure or measures being used. This approach is applied to the assessment of effect size measures for educational interventions designed to improve student academic achievement. Three types of empirical benchmarks are illustrated: (a) normative expectations for growth over time in student achievement, (b) policy-relevant gaps in student achievement by demographic group or school performance, and (c) effect size results from past research for similar interventions and target populations. The findings can be used to help assess educational interventions, and the process of doing so can provide guidelines for how to develop and use such benchmarks in other fields.