While engineering students are required to complete a number of mathematics courses, some engineering students and practitioners believe that they do not use the mathematics that they learned from their courses in engineering projects. This study investigates engineering students’ use of mathematics through observations of two teams of students working on extensive design projects. The case studies presented in this chapter provide insights into situations when engineering students engage in modeling behavior and also explore ambiguity and precision in engineering design. These insights can inform engineering education as we help engineering students become more aware of the ways that mathematics is used in engineering. Additionally, understanding the ways that mathematics and mathematical thinking is used in professional applications can help us motivate and contextualize mathematics instruction as well as determine what should be taught to students in both college and pre-college settings.