This paper explores two complementary approaches to developing empirical benchmarks for achievement effect sizes in educational interventions. The first approach characterizes the natural developmental progress in achievement by students from one year to the next as effect sizes. Data for seven nationally standardized achievement tests show large annual gains in the early elementary grades, followed by gradually declining gains in later grades. A given intervention effect will therefore look quite different when compared with the annual progress for different grade levels. The second approach explores achievement gaps for policy-relevant subgroups of students or schools. Data from national and district-level achievement tests show that, when represented as effect sizes, student gaps are relatively small for gender and much larger for economic disadvantage and race/ethnicity. For schools, the differences between weak schools and average schools are surprisingly modest when expressed as student-level effect sizes. A given intervention effect viewed in terms of its potential for closing one of these performance gaps will therefore look very different, depending on which gap is considered.