Female student engagement in the STEM fields does not currently match the level at which male students are engaged. Whether it is in high school course enrollment, college, or careers, males outnumber females in terms of enrollment and study of advanced physics and engineering. The purpose of this mixed-methods, phenomenological action research dissertation was to uncover reasons why female students, who have otherwise exhibited individual Advanced Placement Physics-C Potential for success in the study of physics at the advanced level, were not enrolling in the Advanced Topics in Physics Honors course (an AP Physics-C comparative course). The goal of this dissertation was to provide greater opportunities for success for all students and to reduce the current level of gender disproportionality. The purpose of this study was to develop intervention strategies, including action steps to reverse this trend and promote a more proportional, and equitable, enrollment of males and females in advanced physics courses. Quantitative and qualitative data from student participant surveys and follow-up focus group interview sessions involved male and female student participants who exhibit Advanced Placement (or AP) Physics-C Potential. AP Physics-C Potential was used as a factor to determine which female students to target for intervention, with the anticipated result being increased course enrollment. The research revealed that an encouraging message from school counselors, teachers, and peers, delivered along with awareness of individual AP Physics-C Potential can promote an increased level of female student enrollment in advanced physics courses and reduce the phenomenon of gender disproportionality which precipitated this study. [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.]