This multiple-case study examined constraints encountered by early career elementary school teachers as they try to deliver high-quality science instruction. Four first-year teachers, who expressed confidence in their preparation and enthusiasm for teaching science, were asked to reflect on the most effective strategies for teaching elementary science and on the constraints to implementing those strategies. Effective strategies for teaching elementary science included inquiry-based instruction and group work with peers, discrepant events, and integration across other subjects and work with student vocabulary. Three categories of constraints emerged related to planning and delivering high-quality science lessons: access to materials and resources, scheduling and prioritizing science instruction, and challenges associated with early career teaching. The findings provide evidence that factors beyond teacher preparation and teacher self-efficacy continue to negatively impact science instruction in elementary classrooms. Promising avenues for future research in science education include more extensive examinations of educational policy, elementary school leaders' perspectives and valuation of teaching science, and strategies for helping teachers advocate for best science teaching practices within their schools.