This research study examined 394 K-12 teacher interns' incorporation of multicultural content and theory into their teaching practices during a 16-week internship in schools, mostly located within a 30-mile radius of Mississippi State University. The teacher interns had completed all coursework and practicum hours required by their teacher education program. As part of their duties, the mentor teachers evaluated the interns' incorporation of multicultural content and theory, using two indicators from the Teacher Intern Assessment Instrument: "Uses knowledge of students' backgrounds, interests, experiences, and prior knowledge to make instructions relevant and meaningful" and "Incorporates diversity, including multicultural perspectives into lessons". To conduct the study, the researcher used descriptive and causal comparative research designs. Results from the paired- sample T-tests indicated a statistically significant difference between initial and final mean scores for both indicators. The Cohen's d effect size indicated the 16-week internship had a large affect on the scores provided by the mentor teachers for the 394 K-12 teacher interns. Findings from the study indicated improvement in the interns' overall incorporation of multicultural content and theory into their teaching practices. As a result of the findings from this study, the researcher recommended that professors and instructors increase the number of assignments, which provide opportunities for pre-service teachers to plan and demonstrate the incorporation of multiculturalism into their teaching practices. The researcher also recommended that facilitators of professional development from the Office of Clinical Field-based Instruction and Licensure increase the amount of time spent on the incorporation of multiculturalism and diversity. [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/en-US/products/dissertations/individuals.shtml.]