This brief summarizes findings on class size from over 25 years of work on the Tennessee Student Teacher Achievement Ratio (STAR) randomized, longitudinal experiment, and other Class-Size Reduction (CSR) studies throughout the United States, Australia, Hong Kong, Sweden, Great Britain, and elsewhere. The brief concludes with recommendations. The STAR research shows that small classes (15-17 pupils) in kindergarten through third grade (K-3) provide short- and long-term benefits for students, teachers, and society at large. Although all students benefit; poor, minority, and male students reap extra benefits in terms of improved test outcomes, school engagement, and reduced grade retention and dropout rates. Differing formulas for counting students and teachers are a major impediment to understanding and using small classes correctly: a pupil-teacher ratio (PTR) is a division problem, class size is an addition problem. The two are not the same, and thus PTR data cannot be used as a substitute for actual class-size data. (Contains 2 tables, 4 resources and 13 footnotes.)