What happens when testing professionals try to agree on what “validity” really means? The Big Validity Game is a playful and thought-provoking discussion for both newcomers and experienced practitioners. Together, we’ll explore key perspectives—from Cronbach and Meehl’s construct validity, to Messick’s unified framework, Kane’s argument-based approach, and contemporary views from Borsboom, Mislevy, Sireci, Zumbo, Lane and Marion, and Solano-Flores; among many others. With interactive prompts, mini-challenges, and live audience polling, participants will reflect on how validity shapes their work across psychometrics, policy, development, and security. By the end, we’ll co-create a shared definition of validity that captures the collective wisdom and lived experience of the ATP community.