This study was conducted by conceptualising, designing and implementing new knowledge at the forefront of body image inflexibility (a construct derived from Acceptance and Commitment Therapy (ACT)) and disordered eating. The study design was adjusted in the light of emergent issues and understandings. The thesis involved a review of the literature which investigated disordered eating and the factors known to influence disordered eating such as body image inflexibility. A critical understanding of the current state of knowledge in this field of theory and practice has been provided. Overall the literature review suggested that body image inflexibility is an important factor to examine when investigating the predictors of disordered eating behaviours. The current study involved the creation and interpretation of new knowledge through original research and advanced scholarship by using a quantitative online survey and latent structural equation modelling (SEM) to test an adapted version of the Tripartite Influence Model with the inclusion of body image inflexibility with 378 adult women. In the method, the range of study design methods and statistical analyses that were considered for this study were outlined. A rationale was provided for the chosen study design and statistical analysis, in order to demonstrate a critical understanding of the methodology of enquiry. The results identified that body image inflexibility represented a single pathway which fully mediated the relationship between women’s body image and their engagement in disordered eating behaviours. Body image inflexibility also fully mediated the relationships between internalisation of the thin ideal and disordered eating and between friend pressure and disordered eating. Perceived pressure to be thin from friends, partners, 15 family and the media also had distinct relationships in the model. Perceived pressure from friends was significantly related to body image inflexibility. Both pressure from partners and pressure from the media had direct relationships with internalisation of the thin ideal and also with disordered eating behaviours. Pressure from family had a direct relationship with body image. Internalisation of the thin ideal and body image had significant mediational roles in the model. Internalisation of the thin ideal fully mediated the relationship from partner pressure to body image inflexibility, from media pressure to body image inflexibility and from partner pressure to body image. Internalisation of the thin ideal partially mediated the relationship between media pressure and body image. Body image fully mediated the relationships between media pressure and body image inflexibility, and between family pressure and body image inflexibility. Body image partially mediated the relationship between internalisation of the thin ideal and body image inflexibility. The results have highlighted the importance of including body image inflexibility as a mediating variable in theoretical models of disordered eating and as a psychological construct to target in treatment interventions for disordered eating. In the discussion, an independent judgement of issues and ideas within the field of disordered eating and psychological flexibility research and practice was provided. Finally a critical reflection on the current study and its strengths and weaknesses was then outlined.