Class size has a long history of research. To date, there is high quality evidence from causal studies suggesting that smaller class size yields short and long-term benefits for students. The understanding on how smaller class size achieve their benefits, i.e., the mechanisms, though, is less clear. Using data from the Head Start Family and Child Experiences Survey (FACES) 2009 cohort, I used propensity score techniques to investigate the effects of class size on behavioral outcomes for children who enrolled in Head Start for the first time in 2009, in full-day classrooms with predominantly 4 and 5- year olds. I also studied the role of teacher-student interactions in the classroom as a potential mediator of the above relationship. I found that smaller class sizes (17-18 children per class) had a very small but non-statistically significant effect (+0.10 S.D.) on student behavioral outcomes over comparison class sizes (19-20 children per class). I also found a statistically significant effect of smaller class sizes on the quality of teacher-student interactions in the classroom (+0.33 S.D.). This effect was driven mainly by a sub-component of the teacher-student interaction scale, namely, classroom organization (+0.42 S.D.). The findings did not rule out the hypothesis that the quality of teacher-student interactions in the classroom may be a potential mechanism by which smaller class size achieve their effects on students. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]