This basic qualitative research study highlighted the specific strategies, techniques, and processes of nine secondary teachers who differentiate instruction in a secondary modern foreign language course to successfully meet the needs of non-native mixed-ability students. The nine participants from across the United States participated in semi structured interviews and provided artifacts that showed how they used data, planned their lessons, and described their experiences with differentiated instruction. Additionally, the teachers explained their challenges and successes with differentiation and described the training they received and deemed effective. Data analysis discovered that effective foreign language teachers planned differentiated lessons based on student needs and interests and used data-driven decision making to develop lessons that used multiple instructional strategies. These teachers use frequent and multiple assessments as well as multiple and flexible grouping strategies. The findings from the study indicated that successful foreign language teachers modify existing differentiated activities and strategies to suit their needs and establish classroom communities by open discussion and collaboration. The findings also showed that effective foreign language teachers collaborate with colleagues to plan and learn more about differentiated instruction and use reflective practice to improve DI lessons. Because of the use of differentiated instruction in their classes, these teachers indicated that they have built student autonomy and an improved student perception of their classes. Finally, the study indicated that teachers need additional time to plan their lessons and learn more about differentiation and need differentiation training that is specific to foreign language instruction. [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.]