Because classroom size reduction (CSR) and standards based testing were implemented at the same time in California during the mid-1990s, it is difficult to isolate the effects of classroom size on outcomes from the effects of curriculum changes. As a result, the main comparison in this paper is very specific--the effect of reduced sized kindergarten classrooms on test performance in second grade. The main result of the paper is that the effect of classroom size reduction is small, especially when compared to the other determinants of student achievement at the second grade level. There is a very small effect of classroom size reduction on student achievement in reading and math, and no effect in language and spelling. Of the two offsetting effects of CSR controlling for spending--the direct effect of smaller classes and the indirect effect on teacher quality, the class-size effect is larger. Over time, the negative effect on teacher quality is reduced as new teachers gain experience and credentials. (Contains 3 figures and 2 tables.)