Survey research examined the extent to which teacher knowledge and training in differentiated instruction (DI) predicted student achievement. The research examined the independent variables (IV) DI knowledge, training, and instructional strategies for effect on dependent variables (DV) ELA and Math student achievement scores. Using a survey developed for this research, the IVs measure the degree of teacher knowledge of DI ("knowledge"), and the degree of teacher training in DI ("training"). The DVs used in a regression analysis were New York State Math and ELA proficiency levels. The regression analysis showed that there was a significant positive effect of DI knowledge for L2 Grade 4 Math, L2 Grade 6 ELA, and of DI training for L2 Grade 4 Math and L2 Grade 6 ELA. These results suggest that teacher knowledge and training about DI may be more effective for students at lower proficiency levels. There were several instances where significance rates were slightly greater than .05, suggesting further study may find an effect on higher achievement levels as well. These findings validate the effectiveness of teachers with higher levels of DI knowledge and training and could be useful to colleges and universities training candidates for classroom instruction. School administrators may also be guided by the findings when developing instructional models, allocating resources, and planning professional development. [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.]