Having good teachers in the classroom matters, and in recent years better data have helped researchers more accurately measure individual teachers' effects on their pupils. A novel RAND study goes further, finding that teachers' effectiveness (or lack thereof) ripples out beyond their own students and affects the educational experiences of their pupils' future peers. The research also found that ignoring a teacher's effect on his or her students' future peers can underestimate a good teacher's value by anywhere from 30 to 90 percent, depending on which assumptions are made about the dynamic nature of the comparisons and the precise definition of the peer group. To better understand how a teacher's impact may extend beyond the students in her classroom, a new RAND study, "Does Helping John Help Sue? Evidence of Spillovers in Education," highlights the potential spillover effects of a good teacher by separating out the effect of a student's peers' test scores from the effect of other aspects of peers' backgrounds. This research brief summarizes findings from this study and suggests future research questions that could further broaden the understanding of how good teachers can indirectly have a positive influence on their students' future peers.