This doctoral thesis is based on the following articles: 1. Assessment of lung volumes in children and adolescents: comparison of two plethysmographic techniques. Halvorsen T, Skadberg BT, Eide GE, Røksund OD, Bakke P, Thorsen E. Clin Physiol Funct Imaging 2005; 25: 62-8. 2. Pulmonary outcome in adolescents of extreme preterm birth: a regional cohort study. Halvorsen T, Skadberg BT, Eide GE, Røksund OD, Carlsen KH, Bakke P. Acta Paediatr. 2004; 93: 1294-300. 3. Better care of immature infants, has it influenced long-term pulmonary outcome? Halvorsen, T, Skadberg BT, Eide GE, Røksund OD, Markestad T. Acta Paediatr. 2005, Accepted for publication. 4. Characteristics of asthma and airway hyper-responsiveness after premature birth. Halvorsen T, Skadberg BT, Eide GE, Røksund OD, Aksnes L, Øymar K. Pediatr Allergy Immunol. 2005; 16: 487-494. 5. High-resolution CT in ex-premature children. Aukland SM, Halvorsen T, Fosse KR, Daltveit AK, Rosendahl K. Am J Roentgenol. 2005, Accepted for publication.