abstract: This action research addressed teacher effectiveness in supporting students’ critical thinking skills by implementing differentiated instructional strategies in eight 3rd- and 4th-grade, self-contained, inclusive classrooms. This study addressed how third- and fourth-grade teachers perceived their instructional effectiveness, how differentiated instructional strategies influence third- and fourth-grade teachers, and how third- and fourth-grade teachers make further use of differentiated instruction to support students’ critical thinking skills across cultures, linguistics, and achievement levels to increase student achievement. Out of the enrollment in a southwest Phoenix elementary school, there was a 35% mobility rate; 76%, free and reduced lunches; 35%, Spanish-speaking homes; 10%, ELL services; and 10%, special education. The school was comprised of 52 certified teachers, out of which there were five related arts teachers, and four teachers who served gifted and special education students. Participants included all eight third- and fourth-grade teachers, 75% female and 25% males; 75% identified as Caucasian and 25% Hispanic/Latina, middle-class citizens. Professional development training was provided to these eight individual teachers during four months on differentiated instructional strategies to support students’ critical thinking. At this study’s beginning, these teachers perceived an obstacle to supporting students’ critical thinking as they struggled to learn new curriculums. Persevering through this challenge, teachers discovered success by implementing design-thinking, developing students’ growth mindsets, and utilizing cultural responsive teaching. These teachers identified three differentiated instructional strategies which impacted students’ academic progress: instructional scaffolds, collaborative group work, and project-based learning. Building upon linguistic responsive teaching, cultural responsive teaching, and Vygotsky’s socio-cultural theory, teachers revealed how to support students’ critical thinking through the use of graphic organizers, sentence frames, explicit instructions, growth mindsets, cultural references, and grouping structures. In addition, the outcomes demonstrated teachers can make further use of differentiated instruction by focusing on instructional groups, teachers’ mindsets, and methods for teaching accelerated learners. This study’s results have implications on teachers’ perception toward using differentiated instructional strategies as a viable method to support the multiple ways all students learn. / Dissertation/Thesis / Doctoral Dissertation Leadership and Innovation 2018