Despite calls from national initiatives for increased STEM learning in the K-12 classroom, addressing STEM in teacher preparation programs has received minimal attention (Brown, 2012). As a result, most courses remained siloed within specific content areas, with little integration (Bergman & Morphew, 2015). This has led to many elementary teachers' apprehension about teaching STEM subjects (Rittmayer & Beier, 2009), and a lacking an understanding of the integrative nature of STEM learning due to the unclear definition (Saito et al., 2016). This case study investigated how a Midwest university's pre-service teachers (PSTs) conceptualized trans- and interdisciplinary STEM connections when designing an integrated STEM project for elementary-aged students involving a localized environmental phenomenon aligned with a grade level, Common Core State Standards for Mathematics (CCSSM) and Next Generation Science Standard (NGSS), to address this issue. The analysis used an adapted ISTEM rubric (Mentzer & Wang, 2019) examining the integrated STEM project (n=13) and deductive coding for the investigation of the individual PST reflections (n=8) on their experiences designing the project to observe these perceived conceptualizations. The findings showed that PSTs consider trans- and interdisciplinary connections essential to integrated STEM learning opportunities according to the localized phenomena selected, the 21st century skills infused into the design-learning, such as collaboration and innovation, and the appropriate alignment to grade-level content standards. The PSTs' reflection submissions supported the conceptualization of trans- and interdisciplinary to include essential components with PSTs citing the group brainstorming opportunities to provide elementary-aged students with grade-appropriate, collaborative opportunities that would resonate with students. However, these findings highlight the complexity of capturing conceptualizations of trans- and interdisciplinary STEM learning approaches and research would benefit from further investigations. [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.]