This article summarizes research on the effect of testing on student achievement as found in English-language sources, comprising several hundred studies conducted between 1910 and 2010. Among quantitative studies, mean effect sizes range from a moderate d [image omitted] 0.55 to a fairly large d [image omitted] 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 frequency also strongly and positively affects achievement. Survey studies produce effect sizes above 1.0. Ninety-three percent of qualitative studies analyzed also reported positive effects. (Contains 5 notes, 1 figure and 10 tables.)