The purpose of this qualitative study was to determine if teacher perceptions of dyslexia accommodations affect the proper implementation of accommodations in the testing process with a focus on formative and summative assessments. The following research question and sub-questions guided the study: What are the perceptions of teachers regarding formative and summative testing accommodations as they relate to academic achievement for dyslexic students? What are teachers' perceptions regarding how and whether accommodations impact student achievement? How frequently do teachers offer accommodations while assessing student growth during assessment activities? This qualitative, case study used middle school teachers from a private Christian school with experience in teaching students with dyslexia through questionnaires and interviews to determine their perceptions of testing accommodations for formative and summative assessments. Research questions and sub-questions were analyzed through the lens of the validity theory. Five themes and seven sub-themes emerged from this study including: (1) knowledge of dyslexia with an emerging subtheme of the purpose of accommodations; (2) accommodations with the emerging subthemes of types of accommodations, formative testing accommodations, and summative testing accommodations; (3) impact of accommodations with the subtheme student success; (4) test validity with a subtheme of fair/unfair; and (5) accommodations in action with a subtheme of teacher use and student use. Overall, this study contributes to the existing literature on how teacher perceptions of dyslexia accommodations affect the proper implementation of accommodations in the testing process with a focus on formative and summative assessments. [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.]