In spite of the well-demonstrated benefits for patients, 20-40% of the patients with COPD who are referred to a pulmonary rehabilitation programme do not complete treatment. The aim of this thesis was to investigate the association of illness perceptions and treatment beliefs with treatment adherence and treatment outcomes, using the Common Sense Model (Leventhal et al., 1980) and the Necessity-Concerns Framework (Horne, 2003) as theoretical background. Our studies showed that attendance, but not drop-out during pulmonary rehabilitation was positively related to patients’ belief that their condition could be improved by treatment. Results also demonstrated that a positive evaluation of the outcomes of treatment led patients to adopt more optimistic illness perceptions, in particular the perceived controllability and variability of the symptoms and the consequences of the illness. With regard to specific ideas about exercise, results showed that concerns about the possible adverse consequences of training were negatively related to patients’ exercise test performance at baseline. For patients with mild-to-moderate COPD, these concerns also had a negative impact on response to training. We conclude that patients’ perceptions of illness and treatment are important for understanding adherence and treatment outcomes, and should be considered as possible treatment objective for the individual patient. ; Promotores: A.A. Kaptein, K.F. Rabe, Co-promotor: M. Scharloo ; With Summary in Dutch ; Netherlands Asthma Foundation, Boehringer‐Ingelheim, GlaxoSmithKline and AstraZeneca