While federal policies such as NCLB, IDEA, and ESSA have placed increased accountability on states to deliver instruction aligned to the grade-level mathematics standards for all students, teaching functional skills to students with significant intellectual disabilities remains a predominant practice. The purpose of this study was to explore mathematics instruction for high school students with significant intellectual disabilities (SID), factors that influenced special education teachers' math instruction and curriculum design at the secondary level, and to determine barriers that may interfere with the delivery of standards-based mathematics instruction. A statewide sample of 75 Illinois secondary special educators who deliver math instruction to students with SID completed a survey that examined their perceptions related to the implementation of national mathematics standards and evidence-based practices in the areas of: (a) knowledge and familiarity with mathematics topics, (b) beliefs about mathematics instruction for students with significant intellectual disabilities, (c) preparation in mathematics instruction, (d) enactment of mathematics content and instructional strategies, and (e) challenges to teaching effective mathematics instruction. Respondents reported mostly teaching functional math skills, such as time and money to students with SID. Further, special education teachers determined the curriculum in isolation, with limited opportunities for collaboration. The findings also revealed that special education teachers endorsed stronger content knowledge of curriculum adaptations than mathematics subject matter. Though no predictors were found that contributed to the content taught, there was a large, positive correlation between how often each content area was taught to how important teachers rated the content area for their students with SID. Implications for research and practice 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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]