For the diagnostic work-up of childhood airway diseases lungfunction measurements play an important role. As childhood represents a period of continuous development and growth, not only is body size and weight increasing and body composition changing with age, but also is the lung developing both in size and airway differentiation. Results from lung function measurements in children and adolescents have thus to be related to lung function data which have been determined in a normal healthy age-matched childhood population. The quality of the reference values is therefore crucial for the interpretation of lung function results.