We report on 9002nc progress towards the development of an Action Concept Inventory (ACI), a test that measures student understanding of action principles in introductory mechanics and optics.The ACI also covers key concepts of many-paths quantum mechanics, from which classical click here action physics arises.We used a multistage iterative development cycle for incorporating expert and student feedback into successive revisions of the ACI.The student feedback, including think-aloud interviews, enabled us to identify their misconceptions about action physics.