Many districts and schools are having difficulty implementing Multi-Tiered System of Supports (MTSS) in school settings. This quantitative study set out to examine the stage of MTSS implementation schools are at and identify factors from the implementation science framework that account for the different reported student outcomes related to MTSS implementation. The results of the responses from 135 school principals in California revealed, most schools were in the initial implementation stage. While implementation drivers of competency and organization were significant predictors of MTSS student outcomes, the leadership driver was found not to be a significant predictor of student outcomes. Very few schools are using a formal evaluation of implementation of MTSS practices or involving parents in the planning and evaluation of MTSS. Additionally, the results revealed that schools have most elements of evidence based practices (EBPs) for academic difficulties, but less EBPs are in place for students who are either at-risk or have emotional behavioral difficulties. A majority of the respondents indicated that there is majority buy-in among the school staff regarding implementation of MTSS. Based on the results of this study, recommendations for Policy, Practice and Research were thematically presented, in the areas of 1) Family engagement, 2) Program evaluation, 3) Budget allocation, and 4) Technical assistance to effectively implement MTSS for sustainability and student outcomes. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]