Low mathematics achievement in the United States has led to the recent advances in the development and evaluation of Tier 2 mathematics interventions designed to close gaps in mathematics that exist at school entry and prevent these gaps from widening. Because Tier 2 mathematics interventions are generally effective for improving mathematics outcomes for most, but not all, at-risk students, recent research has been focused on understanding for whom these interventions are most effective by examining student-level variables that may impact intervention effectiveness. One cognitive variable of particular interest has been executive function (EF). Using extant data collected for the Fusion Efficacy Project (Clarke et al., 2015), this dissertation examined: (1) whether there was differential response to the Fusion, a Tier 2 small-group first-grade mathematics intervention program, as a function of pre-intervention EF, measured by the Head Toes Knees Shoulders (HTKS) Task and (2) whether these differential effects varied by group size. Participants included first-grade students (n = 459) within classrooms (n = 53) randomly assigned to one of two treatment conditions (a small group of two or five students) or the business-as-usual control condition. Proximal and distal mathematics outcome measures were collected at pretest and posttest. Results of the moderation analyses indicated that, compared to the mathematics gains of students in the control condition, gains in mathematics of students in the intervention condition did not differ as a function of pre-intervention EF. However, moderation analyses did reveal a differential response based on group size for one of the four mathematics outcome measures, such that students with lower initial EF gained greater benefit from the intervention delivered in the smaller group (2:1) compared to the larger group (5:1). Results are discussed in the context of implications for future research and practice. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]