We discuss the evolution of an instructional unit for supporting prospective elementary and middle grades' teachers' conceptions of fractions as measures. We first studied prospective teachers' conceptions of fractions using a task-based written assessment with discrete, linear, and circle representations. We designed an instructional unit that involved systematically introducing and connecting non-standard circles representations to other representations of fractions. We describe the assessments, learning outcomes of the instructional unit, limitations, and future directions.