The 24-item Body Image Flexibility and Inflexibility Scale (BIFIS; Brichacek et al., 2023) measures body image flexibility (i.e., openly experiencing negative body image thoughts, feelings, emotions, and sensations as they arise while connecting with a broader sense of self and personal values) and body image inflexibility (i.e., resisting, or getting stuck in, negative body-related experiences and disconnecting from important areas of life) as two distinct ways of responding to body image threats. The BIFIS was developed for adolescents and emerging adults (aged 11 to 30 years), is free to use, and can be administered online or in-person. This chapter describes the adaptation and development of the BIFIS and provides evidence of its psychometrics. The BIFIS has a hierarchical factor structure (two higher-order body image flexibility and inflexibility factors, each comprising four subfactors) and demonstrates measurement invariance across binary genders and age cohorts, and over time. Internal consistency, test-retest reliability, convergent validity, discriminant validity, and incremental validity support the use of the BIFIS with young people. This chapter includes the BIFIS items, response scale, and instructions for administration and scoring. Logistics of use, including permissions, copyright, and contact information, are also provided.