Design research is usually motivated by university members with experience and interest in building theory and instructional designs in collaboration with one teacher. Typically, the teacher is considered as a member of the research team, with the primary responsibility of implementing instruction. However, in this chapter, I describe a Classroom Design Research project that was conducted by a team comprised mostly of classroom teachers. Their goal was to create a stable instructional unit for integer addition and subtraction that they could use to help students learn the topic with meaning. In this paper, I outline the basic tenets of Classroom Design Research, including the instructional theory of Realistic Mathematics Education and how it guided them in designing instruction. I introduce the construct of a classroom learning trajectory and elaborate on it with the integer project. Finally, I argue that Design Research is mutually beneficial for researchers and teachers. The team of teachers that participated in Design Research embarked on a unique professional development experience, one in which they engaged in practices that supported a new way of preparing their instruction. Reciprocally, the teachers' unique craft and pedagogical content knowledge shaped the integer instruction theory in unique ways that do not occur in typical Design Research.