Purpose: Framed by the Concerns-Based Adoption Model, this study investigates female, ethnically diverse preschool teachers’ concerns regarding Science, Technology, Engineering, and Mathematics (STEM) teaching within their participation in a STEM-themed professional learning program and their adoption of STEM content and resources therein. Design/Approach/Methods: An exploratory multiple-case study was conducted with five female, ethnically diverse preschool teachers in Hong Kong. Data were primarily collected through online semi-structured interviews, supplemented by pre- and post-surveys, reflection worksheets, and written peer feedback. Thematic analysis was utilized for data analysis. Findings: Our study revealed five major concerns regarding STEM teaching among female, ethnically diverse preschool teachers, including (1) no confidence in STEM teaching affected by gender bias and minority stereotype, (2) traditional teaching philosophies influenced by Chinese Confucian culture, (3) lack of culturally and contextually appropriate pedagogical strategies and resources, (4) a constrained curriculum structure and limited time and space for STEM activities, and (5) the negative impact of accountability. These concerns were intertwined and created a complex net that affected teachers’ STEM teaching practices. Originality/Value: The study emphasizes the importance of considering sociocultural and organizational contexts and the interplay among various concerns of teachers when developing STEM professional development programs for female, ethnically diverse preschool teachers. Copyright © 2025 The Author(s).