Class size reduction policies have been widely implemented around the world in recent years. However, findings about the effects of class size on student achievement have been mixed. This study examines class size effects on fourth-grade mathematics achievement in 14 European countries using data from TIMSS (Trends in International Mathematics and Science Study) 2011. We employ quasi-experimental methodology (i.e., instrumental variables and regression discontinuity) to facilitate causal inferences of class size effects. Although we find some evidence of class size effects in Romania and the Slovak Republic, overall there are no systematic patterns of class size effects across countries. The results indicate that in most European countries class size reduction may not improve mathematics achievement in fourth grade.