What determines student achievement? The usual approach is to think of achievement as the output of an educational production function. Inputs into this educational production function include teacher quality, class size, resources, peer effects (possibly positive spillover effects and negative disruption effects), and past achievement, since achievement builds on past knowledge. This research focuses on the effects of class size on achievement. This area has been widely studied in both labor economics and education. Somewhat surprisingly, the estimates are relatively mixed. The authors use high-quality administrative data on Greece to first show that there does indeed seem to be a hump shape in the data. Following this, they estimate a relationship between class size and achievement while carefully dealing with issues of endogeneity of class size. The authors show that class size does matter and that the linear specification form used in past work may be why past results were mixed. The authors use estimates to assess hiring and firing and the marginal cost of adding a class. Their estimates here are in line with actual teacher salaries. Finally, in Greece, as in much of the rest of the world, teachers unions are a powerful force to be reckoned with. Their power is demonstrated not only by the wages they set but by their ability to fire teachers at will. [This research brief is based on Desire Kedagni, Kala Krishna, Rigissa Megalokonomou, and Yingyan Zhao, "Does Class Size Matter? How, and at What Cost?," NBER Working Paper no. 25736, April 2019 (ED598832).]