Eating disorders (EDs) are serious psychiatric illnesses that disproportionately affect young women1, 2. Early intervention, when individuals show some signs and symptoms, but before behaviors become more automatic and entrenched can mitigate significant morbidity and mortality3-6. However, there are limited early ED interventions and ED treatment broadly suffers limited access and engagement7-10. Digital interventions have high potential to overcome access barriers and appeal to young people at greatest risk for ED onset2, 11, yet are undeveloped. Many digital interventions simply convert information or in-person activities to an online format or focus on screening and referral, and interventions are primarily based in traditional cognitive-behavioral therapy (CBT)10, 12. Few (if any) have taken a user-centered design approach to development or used gamification, which may increase engagement and learning through operant learning principles. In an initial proof of concept study, we piloted a novel digital, gamified intervention based on Acceptance and Commitment Therapy (ACT)13. Rather than the typical approach to the body weight concerns that underlie EDs, the intervention used gamification to experientially train body-image flexibility (BIF) (i.e., the ability to have distressing thoughts/feelings about the body, without unnecessary attempts to avoid or escape these experiences, and pursue other personally meaningful values or goals). This is a new direction with preliminary support, and would be a paradigm shift in early ED intervention, which has focused on changing or eliminating body-image distress. This Phased R61/R33 (NIH Stage 1A-B) takes a user- centered design and experimental therapeutics approach to further develop and optimize the intervention for young women in the US with ED symptoms. In the R61 Phase of the study, draft sessions are built with streamlined content, and enhanced graphics and interactive features, and we conduct iterative user testing to maximize acceptability and impact on BIF (Aim 1). We then use a multiple baseline experiment across participants to test the effect of the intervention (“FlexED”) on BIF, our targeted mechanism of change, and establish treatment dose (Aim 2). During the R33 Phase, we conduct a pilot RCT comparing FlexED to an online educational control. We test the effect of the intervention on BIF and the associated clinical benefit, and assess whether the intervention results in decoupling of body-image distress and behavior as an additional test of our mechanism of change (Aim 1). We also test a virtual body-Behavioral Approach task as an assessment of BIF in a personally meaningful or valued context that may be used in future investigations (Aim 2). Finally, we assess the acceptability of the final FlexED intervention, as indicated by retention (Aim 3). This proposal prepares for a larger trial with longer-term follow up, with the ultimate aim of establishing a cost-effective, widely available ...