The purpose of this convergent parallel mixed methods study was to understand how being both Black and a girl impacted the mathematical and STEM experiences of Black adolescent girls enrolled in informal STEM learning programs and Black adolescent girls who are not enrolled in informal STEM learning programs. Using Critical Race Theory, this study focused on the intersectionality of the Black girls' mathematical identity, gender identity and racial identity. This study investigated how rising 6th through 8thgrade Black adolescent girls (N=13) described their mathematical and STEM experiences and how their experiences shaped their mathematical and racial identities. Using journals, surveys, and semi-structured interviews, thematic analysis revealed how Black girls' mathematical identity can be impacted by the support or lack of from their teachers, parents, and peers, as well as the influence participating in informal STEM learning programs has on the development of their mathematical identity and STEM interest. Results indicated that adolescent Black girls enrolled in informal STEM learning programs are more likely to have a strong and positive mathematical identity compared to adolescent Black girls not enrolled in informal STEM learning programs. Further findings show that middle school Black girls have high racial identities and believe race is essential to their identity, however their perceptions about Black adults being good in mathematics is skeptical. Implications will highlight the importance of having Black girls participate in informal STEM learning programs and recommendations for informal STEM learning programs to expand and improve the mathematical and STEM experiences for Black girls. [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.]