This study investigated the experiences and perceptions of science teachers and students regarding the implementation and effectiveness of Differentiated Instruction (DI). It also aimed to determine its impact on student learning outcomes measured by the biology End-of-Course (EOC) test. A convergent, parallel, mixed method of research was utilized. The qualitative part involved a phenomenological approach which focused on individual beliefs, experiences and perceptions of teachers about DI. Qualitative data were gathered using personal interviews and were triangulated by looking at the teachers' lesson plans. On the other hand, the quantitative part was focused on determining the effect of DI on student learning outcomes. The End-of-Course (EOC) scores of students in Spring 2015 when students were exposed to DI were compared with those of students in Spring 2014 when students were not yet exposed to DI. Personal interviews with six science teachers and survey results from 65 biology students revealed that teachers and students alike have positive perceptions of DI. The following major themes emerged from the qualitative part of the study: a) Differentiated instruction improves student engagement and academic performance in class; b) Differentiated instruction motivates the students; c) Differentiating by choice is the most common way to differentiate; d) Administrative support has major influence in the implementation of differentiated instruction; e) Implementation of differentiated instruction increases teacher efficiency; and f) Differentiated instruction requires more time and creativity. When tested if students exposed to DI perform better on the EOC than the students who were not, the Analysis of Covariance (ANCOVA) resulted to a p-value of 0.12. The proficiency pretest was taken as a covariate in order to make the two groups statistically comparable. The p-value suggests that at 5% level of significance, the DI group did not perform significantly higher than the non-DI group (p>0.05). This means that although the DI group performed higher in the EOC as suggested by t-test results (p-value=0.046), their exposure to differentiated instruction did not contribute to their higher EOC scores. [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.]