Here we report on the development process of the Inquiry Oriented Instructional Measure (IOIM), an instrument for scoring a lesson along seven inquiry-oriented instructional practices. The development of the IOIM was a multi-phase, iterative process that included reviewing K-16 research literature, analyzing videos of classroom instruction, and pilot testing. This process resulted in the identification of instructional practices that support the successful implementation of inquiry-oriented instruction (IOI) at the undergraduate level. These practices, which comprise the IOIM, provide an empirically grounded description of IOI. In addition, the IOIM provides a rubric for evaluating the degree to which an instructor's classroom instruction is reflective of these practices. As a proof of concept for the IOIM, we present the results of a pilot test -- in which data from a large professional development program designed to support undergraduate mathematics instructors in implementing inquiry-oriented instruction was scored using the IOIM.