Mentalisation, or reflective functioning, is integral to emotional well-being and social communication. Poor mentalisation has been associated with poor mental health, including eating disorders. However, research into how mentalisation interacts with psychosocial factors associated with the development of an eating disorder in non-clinical samples has not been explored. Additionally, how far mentalisation in the parents of adolescents with eating disorders is modifiable, and whether changes in parent mentalisation can impact treatment outcomes for these adolescents, have not been explored. The overarching aims of this thesis therefore are to explore further the roles that mentalisation, including parent mentalisation, plays in the development of eating disorders and in their treatment outcomes, and to understand the usefulness of mentalisation as a therapeutic target for adolescents with eating disorders. To do so, three studies were conducted: a cross-sectional survey-based school study, secondary data analysis on families receiving family therapy, and a prospective observational study of an NHS parent intervention. The results suggest that aspects of mentalisation, particularly self-mentalisation, are strongly associated with disordered eating behaviours, both in clinical and community adolescent samples. Additionally, mentalisation appears to correlate with a variety of eating disorder risk and protective factors. Finally, improved parent mentalisation for this population of parents is possible through both family and parent-focused interventions, but parent mentalisation change is not a direct predictor of treatment outcomes. Areas for future research include use of longitudinal designs to examine how mentalisation and eating disorders develop together through adolescence, the use of a wide range of tools to measure parent and adolescent mentalisation, including the use of Video-feedback Intervention to promote Positive Parenting, and the integration of mentalisation techniques into prevention and treatment interventions.