The purpose of this dissertation was to extend the literature base for teaching students with intellectual and developmental disabilities (IDD) mathematics and self-determination skills using an evidence-based practice called modified schema-based instruction (MSBI). Chapter one provides the rationale for teaching both mathematics and self-determination skills to students with IDD, highlighting the correlation between both skill sets and positive post-secondary outcomes. This chapter presents the rationale for conducting this dissertation, which extends from prior MSBI research and is grounded in educational theory. To answer the five proposed research questions, this dissertation uses a mixed methods embedded experimental design. This dissertation follows mixed methods publishing recommendations by presenting the findings across multiple manuscripts (i.e., chapters). Chapter two presents the findings from the quantitative single-case design exploring the effects of teacher- and paraprofessional-delivered MSBI targeting personal finance mathematics word problems by students with IDD in a small group setting. Results indicate a functional relation and strong effect sizes between MSBI and learned mathematics skills of students with IDD, further adding to the extant literature base of positive student outcomes produced during MSBI instruction. Chapter three presents the findings from the integrated mixed methods results exploring "how", "why", and "under what conditions" MSBI "works" for teaching mathematics to students with IDD through a rigorous evaluation of fidelity of implementation. Findings provide important contextual information for how differentiated instructional delivery of MSBI can impact student outcomes; additionally, results identified six influences impacting instructional delivery and student outcomes, adding novel information for "under what conditions" MSBI is effective in teaching mathematics to students with IDD. [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.]