Teacher assessment literacy has become a central priority across many educational systems in North America and elsewhere in response to growing accountability demands. Although many scholars have aimed to measure teacher assessment literacy, recent research has identified that current assessment literacy instruments do not fully reflect current transformations in the assessment landscape and remain predicated on dated standards for teacher classroom assessment practice. Given significant shifts in classroom assessment over the past 20 years, the purpose of this article is to construct a reliable instrument reflective of contemporary assessment practices and contexts. Specifically, this article describes our instrument development process including construct validation and reliability testing with more than 400 teachers. The result of this research is the Approaches to Classroom Assessment Inventory, which can be used by researchers and practitioners to support teacher assessment literacy in relation to the current accountability framework evident across educational systems.