Computer science (CS) and computational thinking (a problem-solving process used by computer scientists) teach students design, logical reasoning, and problem solving--skills that are valuable in life and in any career. Computational thinking (CT) concepts such as decomposition teach students how to break down and tackle a large complex problem. Engaging in CT practices like debugging (finding and fixing errors), persevering, and collaborating helps to prepare students to be the innovators and developers of new technologies. Elementary educators see the need and benefit of CS education but struggle to fit something new into the already packed elementary school day. While some schools have started robotics and computer science after-school clubs, these do not address the need to educate ALL students in CS. To provide equitable access to CS education, instruction needs to occur during the regular school day and in the classroom. The authors sought to address this problem by developing and implementing interdisciplinary STEM + Computer Science Problem-Based Learning (STEM+CS PBL) units in grades 3-5. CS concepts are integrated with content already being taught, thus eliminating the need for an additional block of instructional time. Each unit centers on a real-world problem, integrates multiple content areas, and includes a culminating performance task/product in which students demonstrate and apply their understanding of the concepts learned. The problem provides context and gives students a purpose for learning content area benchmarks. This article examines one of their five-week-long STEM+ CS PBL units, "Non-Native Invasion," which engaged fourth-grade students with tackling the real-world problem of the invasive Burmese python in the Everglades and required students to design a computer simulation to model and predict how its introduction to the ecosystem affects native populations.