The authors describe the development and validation of the Avoidance and Fusion Questionnaire for Youth (AFQ-Y), a child-report measure of psychological inflexibility engendered by high levels of cognitive fusion and experiential avoidance. Consistent with the theory underlying acceptance and commitment therapy (ACT), items converged into a 17-item scale (AFQ-Y) and an 8-item short form (AFQ-Y8). A multimethod psychometric approach provides preliminary support for the reliability and validity of the AFQ-Y and AFQ-Y8. In 5 substudies, 3 samples (total N = 1369) were used to establish (a) item comprehension (n = 181), (b) initial item selection (n = 513), (c) final item reduction and development of a short form for research (n = 346), (d) comprehensive psychometric evaluation of the AFQ-Y and AFQ-Y8 (n = 329), and (e) convergent and construct validity for both versions of the AFQ-Y. Overall, results suggest that the AFQ-Y and AFQ-Y8 may be useful child-report measures of core ACT processes.