The purpose of this mixed-methods study was to explore the impact of informal STEM education on underrepresented middle school girls. The study took place within the context of the EUREKA! summer program, a collaboration between Drexel University's Department of Civil, Architectural, and Environmental Engineering and Girls Inc. of Philadelphia and Southern New Jersey. EUREKA! provides five summers of cost-free summer STEM programming to underrepresented minority girls enrolled in public and private schools in the Philadelphia region between eighth and twelfth grades. This research specifically examined the impact of the ninth-grade summer program on participants' attitudes toward STEM and 21st-century learning skills over the course of the summer and their ninth-grade school year. The research questions addressed in this study are (1) Is there a statistically significant increase in EUREKA! summer program participants' STEM attitudes over time? (2) Is there a statistically significant increase in EUREKA! summer program participants' 21st-century learning attitudes over time? (3) How do EUREKA! participants describe their STEM attitudes and interest over their ninth-grade year? To answer these questions, the researcher administered a survey and conducted focus groups for a cohort of 15 EUREKA! participants in early July before the ninth-grade summer, in late July after the ninth-grade summer program, during March of the ninth-grade school year, and in June at the end of the ninth-grade school year. The researcher employed a pragmatic framework for data analysis focused on the practical implications of the study's findings. [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.]