Amongst psychiatric illnesses, eating disorders (EDs) are notoriously difficult to treat and have a high mortality rate. The average duration of an ED is 6 years and for a majority of ED patients, the disorder will become chronic. Comorbid personality pathology such as negative core beliefs and early maladaptive schemas (EMS) are strongly related to ED severity and chronicity. Enhanced cognitive-behavioural therapy for eating disorders (CBT-E) is used as the first line transdiagnostic treatment for EDs. However, CBT-E is mainly symptom-focused and does not tap into these underlying core beliefs and EMS. Given the limited treatment effects of existing ED treatments, and the importance of comorbid personality pathology, there is an urgent need to examine more effective treatments for EDs. Group-schematherapy (GST) overcomes the limitations of CBT-E and preliminary results for treatment-resistant EDs are promising. However, robust evidence regarding the clinical and cost-effectiveness of GST for patients that do not benefit from CBT-E is not yet available. The central aim of this project is to investigate the clinical and cost-effectiveness of GST for EDs in patients with comorbid personality pathology, who do not show a clinically significant response in the first phase of CBT-E. This is relevant and important as studies examining the effectiveness of GST for EDs are scarce. This project is a joint research initiative of three academic centers (Dutch Universities), four large nation-wide mental health organizations, and two foundations for client empowerment and participation. Eligible patients will be randomized to either GST or continuation of their CBT-E treatment after failing to show a significant treatment response in the first phase of CBT-E. Based on encouraging findings from previous studies and our own pilot data, a statistically and clinically significant better outcome in terms of ED symptoms, negative core beliefs, EMS, schema modes, and quality of life is expected in the GST group compared to the CBT-E group. GST is also expected to be more cost-effective compared to CBT-E as GST may in the long run prevent chronicity in terms of long treatment trajectories and delayed recovery. Finally, with the proviso of good results for GST, we will disseminate and implement GST in the standard of care for EDs. This project thereby has great potential to improve clinical and cost-effectiveness of treatment for chronic EDs.