Evidence-based mathematics interventions are critical for supporting students with mathematics difficulties. In research and practice, collecting implementation fidelity is important for ensuring that all the core components of the intervention are implemented as designed. Historically, implementation fidelity has been defined as multi-faceted including examinations of adherence, instructional quality, and student engagement, though mathematics intervention studies rarely report on fidelity components outside of adherence. The current study examined relationships between these different components of fidelity and whether they are associated with student mathematics outcomes, intervention group size, and interventionist characteristics within the context of a first-grade mathematics intervention. Findings revealed relationships between components of fidelity with student initial mathematics skill, however no relationship was observed between fidelity components and student mathematics growth. Findings for group size and interventionist characteristics were mixed. Limitations, implications for research and practice, and future directions are discussed. [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.]