Background/Context: The effects of class size on student achievement have gained considerable attention in education research and policy, especially over the last 30 years. Perhaps the best evidence about the effects of class size thus far has been produced from analyses of Project STAR data, a large-scale experiment where students and teachers were randomly assigned to smaller or larger classes within schools. Researchers have also analyzed observational data to examine the effects of class size, but the results have been mixed. Purpose/Objective/Research Question/Focus of Study: It is generally difficult to draw causal inferences about class size effects with observational data because of the omitted variables problem. This shortcoming can be overcome with instrumental variables (IV) methods that are designed to facilitate causal inferences. The present study uses IV methods to examine the effects of class size on reading achievement using data from the 2001 fourth-grade sample of the Progress in International Reading Literacy Study (PIRLS) in Greece. We took advantage of Greece's nationwide rule about maximum class size in elementary schools to construct IV estimates of class size. Population: PIRLS was designed to monitor children's achievement levels in fourth grade worldwide. We used reading achievement data from 2001 in Greece. The sample was a national probability sample of fourth graders. The use of appropriate weights helped us make projections to the fourth-grade student population in Greece in 2001. Research Design: The research design was secondary analysis. We examined whether class size predicts reading achievement for fourth graders in Greece net of student, teacher/classroom, and school characteristics. We used multilevel models to capture the dependency in the data (i.e., students nested within schools). We also used instrumental variables methods to facilitate causal inferences about class size effects. Conclusions: We investigated the effects of class size on reading achievement for fourth graders in Greece in 2001 using rich data from PIRLS. The results produced from the multilevel and the IV analyses were overall similar. Generally, the results indicated a positive association between class size and achievement. However, the association was typically statistically insignificant, especially when teacher/classroom and school variables were taken into account.