Teachers creating instruction for inclusive classrooms prepare for a wide variety of student achievement levels, student preferred learning styles, and personal student emotional needs. Differentiated instruction and formative assessments allow teachers to tailor instruction of state standardized objectives to individual student needs to raising the academic achievement level of each student. This qualitative case study involved exploring teachers' perceptions through interviews and classroom observations of the impact of differentiated instruction and formative assessment implementation. The research sample consisted of eight teachers in inclusive classrooms in four high schools in Sussex County, Delaware. The five emergent themes from the data analysis that described differentiated instruction were building relationships with students, best practices menu of differentiated instruction, teacher collaboration, challenges, and the impact of differentiated instruction. The six emergent themes from the data analysis that described formative assessment use were objectives for teachers and students, best practices menu, feedback, challenges, formative assessment and leadership, and the impact of formative assessment. Findings of the study indicated a positive impact from differentiated instruction and formative assessment on student academic achievement levels, as seen in rising standardized test scores, increased student implicit motivation, and the perception by teachers to meet individual student learning needs effectively. Results of the teacher interviews and classroom observations indicated a well-defined understanding of the tenets of the phenomenon but a lack of consistency in implementing differentiated instruction and formative 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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]