The study summarizes the research literature on the effect of testing on student achievement, which comprises several hundred studies conducted from the early 20th century to the present day. Only quantitative studies are listed here (N studies = 177; N effects = 640). Mean effect sizes range from a moderate d ˜ 0.55 to a fairly large d ˜ 0.88 depending on the way effects are aggregated or effect sizes are adjusted for study artifacts. Testing with feedback produces the strongest positive effect on achievement. Adding stakes or testing with greater frequency also strongly and positively affects achievement. The evidence from a century’s worth of quantitative studies shows the effect of testing on achievement to be moderately to strongly positive.Smaller-scale studies, however, tend to produce stronger effects than do large-scale studies. Those who judge the effect of testing on achievement exclusively from large-sample multivariate studies deprive themselves of the most focused, clear, and precise evidence. Some prominent researchers in economics and education, for example, have claimed that no studies of “test-based accountability” had been conducted before theirs in the early 2000s. But, this list includes 24 studies completed before 2000 whose primary focus was to measure the effect of “test-based accountability.” A few dozen more pre-2000 studies also measured the effect of test-based accountability although such was not their primary focus. Include qualitative and program evaluation studies of test-based accountability, and the count of pre-2000 studies rises into the hundreds.