Abstract Early elementary students are capable of engineering design and the decision making processes that design requires. However, young students often lack the language and ability to clearly express their ideas and justify their decisions, making it difficult to understand their thought processes and abilities. Therefore, in order to know more about the capabilities of young students, we need to better understand the ways in which they make decisions in their design. We address the research question: What resources and evidence do first grade students use to make design decisions? We analyzed classroom video data of first grade students working in small groups during an engineering design-based STEM curriculum across three classrooms. For this qualitative study, we focused on the final two lessons of the curriculum where the students worked in pairs to design and build a model of an exercise trail for a hamster, based on a set of criteria and constraints given in a series of letters from a client and scaffolded by the teacher. Through this activity, the students were particularly engaged in planning, trying, testing and redesigning to solve the proposed engineering design problem. The video recordings of each pair of students were open coded to identify themes that arose around students’ design decisions as they worked through an engineering design challenge. The coding process focused on what the students said and the actions they took during their team work. Analysis revealed that first grade students made a number of explicit and implicit decisions about their design that were based on a variety of sources. Although they based many of their actions on classroom procedures and teacher directions, some students also were able to incorporate input from their partners. Some pairs negotiated designs based on plans from both students whereas others focused on their own design and did not accept changes from their partners. Most of the students struggled to incorporate all of the criteria and constraints of the problem at once so, at different points in the design process, they focused on different constraints. These students often focused on one or a few aspects of the problem, neglecting the other aspects. However, when reminded by a partner or teacher, they were able to quickly remember key components of the problem and start to look for ways to incorporate that into their design. Other students were able to more systematically handle the constraints of the problem and developed processes to prioritize the components of the problem. Students prior knowledge and ideas also placed a role in the decisions they made, such as how they chose to negotiate with their partner and which criteria they chose to focus on the most. Based on this work, we discuss implications of this work for thinking about early engineering learning and developmentally appropriate engineering design.