This qualitative study examines how FIESTAS (Family Involved in Education, Sociocultural Teaching and STEM) partners (comprised of out-of-school time (OST) educators, university student STEM instructors, and local business representatives) came together to facilitate a local STEM Club located in two dual-immersion elementary schools in the Pacific Northwest. The study addresses my research questions of (1) How do FIESTAS partners engage in STEM Club and why? and (2) What tensions exist for FIESTAS partners in the facilitation of STEM Club, and how do the partners negotiate these tensions? Previous literature on how programs are planned, implemented, and revised in OST is scant. This indicates that the voices of those who actually deliver these programs – those voices which can inform the localized conditions that led to the outcomes of the partnership – are usually left out of the research process. Therefore, data collection for this study explicitly focused on partners engaged in supporting youth in OST STEM education. The study utilized Cultural Historical Activity Theory’s attention to individual voices within a collaborative goal-directed activity. The study’s findings document that FIESTAS partners came into the partnership with particular historically-informed concepts of what STEM education is and what youth might enjoy in OST; these concepts shaped how partners engaged with the partnership. The findings support a growing body of literature that explores the vital roles education partnerships can play in supporting OST STEM learning.