According to the National College Attainment Network (NCAN, 2023), each year, thousands of high school graduates leave billions of dollars in federal student aid unclaimed because they do not complete the Free Application for Federal Student Aid (FAFSA). To holistically investigate this phenomenon, this dissertation accomplished three goals. The first goal was to identify predictors of 11th-grade students' plans to complete the FAFSA. The second goal was to determine reasons why students lacked plans to complete the FAFSA, and the third goal was to identify predictors of FAFSA completion in the fall following high school graduation. Using cultural and social capital theories as a framework, this study considered a constellation of possible predictors of these key outcomes, including student demographic characteristics, family resources, parent behaviors, and school-based sources of support. This study separately considered predictors of FAFSA completion among students initially without plans to complete the application. Using the data from the High School Longitudinal Study of 2009 (HSLS:09), the research objectives were accomplished using descriptive statistics and logistic regression analysis. The results show that male, first-generation, and higher-income students were the least likely to have FAFSA plans. Parents gathering financial aid information through social networks and from institutional and high school sources were positively related to having FAFSA plans, as was having friends planning to attend college. The ability to afford college without financial aid surfaced as the most common reason students lacked plans to complete the FAFSA, and the least common reason was that the FAFSA was too complicated. Finally, males, first-generation students, and higher-income families were the least likely to complete the FAFSA. Neither FAFSA-specific nor other financial aid-related school-based sources of support significantly predicted FAFSA completion. For those students without plans for FAFSA completion, predictors of FAFSA completion were substantively the same, indicating that school-based efforts should be focused on building a college-going climate, even before high school, and tailoring supplemental supports, especially for male and first-generation students. Such supports should prioritize information for parents and student peer learning opportunities focused on the vast array of financial aid options that are accessible only by completing the FAFSA. [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.]