The studies in this thesis focused on two complex psychopathology target groups (personality disorders and underweight eating disorders with co-morbid post traumatic stress disorder) for whom the effectiveness of the treatments then on offer showed that there was still a world to be won. The studies involved pioneering research on the effectiveness of experiential techniques in these groups, and on patients’ and their therapists’ experiences with them. Another respect in which these studies were pioneering is that, so far, there had been no fixed protocols to work with. Listening to all participants – patients and therapists alike – provided a detailed picture of important experiences, thereby helping to better understand why the treatment, training and peer group supervisions did or did not work, and for whom. These experiences also provided very useful advice on how the treatments, performances and trainings could be improved. The qualitative, in-depth findings, helped to better explain some outcomes of the effectiveness study. Experiential techniques help patients reduce symptoms, and experiential learning helps therapists learn a new technique. Finally, pioneering research on trauma treatment for people who were underweight gave new hope to a group of difficult-to-treat patients.