This study presents a conceptual and evidentiary study pertaining to the micro and macro level factors influencing the achievement gap among migrant children from a cross-national standpoint. First, this study provides the most systematic investigation of the immigrant educational paradox theory, which asserts that on average, immigrants' educational outcomes tend to exceed those of non-immigrants. This study examined the validity of the immigrant educational paradox theory cross-nationally for three content areas (mathematics, science, and reading), across the different types of tests, Trends in International Mathematics and Science Study (TIMSS) 2015 and the Progress in International Reading Literacy Study (PIRLS) 2016, across fourth and eighth grades. As immigration is a rapidly-growing global phenomenon, this study ranked countries based on first and second generation immigrants' achievement. Thus, the findings revealed that in some traditional immigration countries, the immigrant educational paradox was discovered, including New Zealand, Australia, England, and Singapore. Further, the immigrant educational paradox was revealed with a high magnitude in all content areas and across both grades in the Gulf Cooperation Council (GCC) countries including The United Arab Emirates, Kuwait, Qatar, Bahrain, Saudi Arabia, and Oman. Overall, the findings indicate that the immigrant educational paradox is a very heterogeneous phenomenon and varies by country, grade, and type of content assessed. Younger students show a larger immigrant achievement gap when compared to eighth graders where the widest gap was in science. The second paper of this dissertation presents a conceptual and evidentiary study about the micro and macro level factors influencing the achievement gap among migrant children from a cross-national standpoint. This research study was able to include micro and macro level variables using TIMSS 2015 grade four to measure first and second generation immigrants' math and science achievement. Hierarchical linear modeling (HLM) was chosen as it estimated assuming that students are nested within schools. HLM control for nesting effects and their standard errors are more accurate than other tools. It underscores that the migrant experience is a heterogenous and multilayered phenomenon. The major predictors for migrant children's performance socio-economic status (SES) and language proficiency, however, their experiences vary based on the host country type. This study draws out policy recommendations for educational systems and school leaders to impact malleable variables in schools and develop more effective mechanisms to reduce educational inequalities among migrant children.Finally, the third study was to compare and contrast student, teacher, and school factors that are associated with mathematics achievement gap in Saudi Arabia and the United States. Using grade four data from the Trends in International Mathematics and Science Study (TIMSS) 2015, this study adds a new variable which is student bullying to examine its association with math achievement in both countries. Hierarchical linear modeling (HLM) was chosen as it estimated assuming that students are nested within schools. It was found that immigrants' performance varied in both countries favoring immigrants in Saudi Arabia yet the opposite is true for the USA. It was found that the gender gap remained visible favoring boys in the USA and favoring girls in Saudi Arabia after controlling for relevant variables. Further, our findings suggest that being a victim of school bullying has a considerable negative effect on student performance in both countries. Hence, school composition and emphasis on academic success seems to be a relevant factor in explaining student performance in both countries. However, the two countries' results differed in the association between students' background, teacher, and school factors. Implications from this study suggest that the bullying and students' gender gap in math achievement are significant issues. [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.]