The goal of this observational longitudinal study is to investigate characteristics and factors associated with the development of Severe and Enduring Eating Disorders (SEED). In this project, the researchers will follow two prospective cohorts of patients with eating disorders (ED), one adolescent (ages 14-17) and one adult (ages 18+), in terms of change in and impact of clinical, psychological, and biological risk factors. Data will be collected at baseline, after treatment, two years after baseline, and thereafter five, 10 and 20 years after baseline. Participants will be asked to undergo a physical examination, leave blood samples, be interviewed, and fill in questionnaires. If the participants are minors, their care takers will also fill in the questionnaires. The study aims to explore how clinical, psychological, and biological risk factors-including comorbidity, personality characteristics, difficulties with emotion regulation (ER), cognitive inflexibility, loneliness, severe ED symptoms, and inflammatory activation-contribute to a chronic course of the disorder.