This Research Note discusses research on class size reduction (CSR). The research on CSR is not definitive but a contested, ongoing debate. The research on CSR has been severely limited by the singular focus on student academic performance, although some studies have sought to expand the conversation by focusing on the impact of CSR on teaching conditions, classroom environment, student learning experience. The research showing 'class size isn't important' has been extensively critiqued for philosophical and methodological weaknesses. Reducing class size in and of itself is not a magic bullet. It's more about what teachers and students can do when class sizes are lower, and a growing number of studies illustrate the way teaching and students' learning experience can change with smaller class sizes.