Mathematical justifications offer an approach to which students deepen and extend their mathematical thinking. This process has the potential to enhance students' conceptual understanding. Mathematical practices and process standards ask students to use critical thinking skills to justify their reasoning, but this is usually seen as proofs in high school geometry. This convergent mixed method research study aimed to increase students' use of writing mathematical justifications and investigate the influence of this practice on mathematical and conceptual understanding. Eleven eighth-grade students participated in the study conducted in the Midwest United States. Data were collected from semi-structured interviews, students' artifacts of written justifications, and pre- and post-survey data. The qualitative data were analyzed by completing three rounds of coding where themes emerged, and the quantitative data were analyzed via descriptive statistics and the Wilcoxon Signed-Rank Test. Findings revealed a significant difference between the first and last justification students wrote. Additionally, students perceived written justifications to be beneficial to understanding the content deeper and the ability to use written expression for explanations as valuable. Results suggested that written mathematical justifications can increase students' conceptual understanding and improve their perception of writing in mathematics. [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.]