With ongoing underrepresentation of women in STEM fields, it is necessary to explore ways to maintain girls' STEM interest throughout elementary and middle school. This study is situated within the context of Designs in STEM (pseudonym), an out-of-school program that engages urban youth in authentic STEM experiences. Participants were 30 girls attending Designs in STEM in grades four and five. Participants were interviewed about their STEM interest, out-of-school versus in-school STEM learning experiences, and how gender relates to STEM success. Several key findings emerged. First, although students' prior school experiences with mathematics resulted in less positive dispositions toward mathematics than other STEM disciplines, their experiences at Designs in STEM revealed that mathematics could be fun and valuable when used for real-world purposes. Second, students found Designs in STEM to be more engaging and inspiring due to the context and pedagogies employed by Designs in STEM instructors. Third, despite observing girls' behavior that was more aligned with academic success, participants still identified STEM advantages for boys. Finally, participants defined success and intelligence in STEM based on speed and tracking. Discussion focuses on the need to consider how school-based mathematics instruction may serve as a barrier to girls' STEM interest and involvement.