Concept inventories (CIs) have become a common form of assessment among undergraduate science, technology, engineering, and mathematics (STEM) educators. These instruments measure conceptual understanding of a variety of subjects. In this chapter, we build on this literature base by presenting a validity argument for a mathematics concept inventory, the Group Theory Concept Assessment (GTCA). This assessment captures conceptual understanding of key topics in introductory group theory. To situate our work, we provide an overview of validity evidence typically provided for undergraduate mathematics CIs. We then share design and data from the GTCA development to instantiate (1) a more varied set of evidence types and (2) a blueprint for a mathematics CI validity argument. Through the validity argument approach we are able to contribute to the mathematics CI literature by illustrating how relevant claims, subclaims, and evidence may be gathered and shared.