Dr. Roy Thewissen is a psychologist and a behavioural therapist. He works at the department of Pain in the rehabilitation center Adelante in Hoensbroek. He introduced and shaped the ACT-model here. As a freelancer he provides trainings, workshops, lectures and supervision in ACT, RFT (Relational Frame Theory) and clinical behavioural analysis. Thewissen got his doctorate at the Maastricht University on the topic of learning processes in exposure therapy for addictive behaviour.