In this paper, we report on the development of an Algebra Concept Inventory (ACI) intended for students transitioning from second- to third-level education. We begin by outlining the work done on concept inventories to date before describing some of the guiding principles of interdisciplinary design teams. Our methodology for developing the ACI is detailed in a step-by-step manner including the formation of the design team, defining the parameters of the ACI, shortlisting items, and piloting. The iterative design process resulted in a 31-item preliminary ACI. We conclude by highlighting aspects of how the interdisciplinary team functioned throughout the process before outlining some potential uses of the ACI which was rolled out during Semester 2 of the 2020-2021 academic year to all higher education institutions (HEIs) in Ireland. The ACI is available for interested practitioners upon request.