The main purpose of respiration is to increase the oxygen content and decrease the carbon dioxide content of the blood in the pulmonary circulation to levels suitable for metabolism in the body. This is done by breathing a gas mixture, usually the atmospheric one. In this paper I shall refer to the whole gas transport as the lung function. In reviewing the development of this complex phenomenon during childhood the following components will be considered1: Lung volumes, mechanical factors involved during a single breath, alveolar ventilation, diffusion, gas exchange (as a criterion of the functional demand on the lungs).