The No Child Left Behind (NCLB) accountability era mandated that teachers accommodate students with varying backgrounds and skill levels. As additional mandates are being placed on educators a growing number of these professionals are assuming teacher-leader roles in order to keep up with the constantly changing demands of their increased responsibilities. With these varying backgrounds and skill levels educators are highly encouraged to implement differentiated instruction in an effort to fulfill each student's potential. This research investigated the instructional challenges and needed supports of secondary teachers when implementing differentiated instruction. To gather data for this study, a qualitative approach, phenomenology, was utilized. With a purposeful sample of secondary educators, the researchers conducted semi-structured interviews to gather data. The participants were asked open-ended questions regarding their own experiences regarding how they perceived the implementation of differentiated instruction. Additionally, an interpretative phenomenological analysis was performed utilizing the data. The study's researcher distilled interview transcripts into themes. The findings from the research revealed that the factors that most influence effective implementation of differentiated instruction were class sizes, preparation and time to plan, monitoring and attending to student behavior, a lack of professional development training, administrative support, and parental support. [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.]