The lung is only partially formed at the time of birth and considerable structural changes take place during childhood, the most important of which appears to be a 6-8 fold increase in the number of alveoli. The process by which new alveoli are developed, i.e. compoundment of septa, segmentation of primary alveoli and alveolisation of respiratory ducts, will be discussed, and their implications related to the probable effects of chronic and s ub-acute diseases of the lung during infancy. The lecture will include photographs of the alveolar patterns of lungs at different ages and in particular photographs of elastic stains showing alveolar ‘nets’. Photographs of acute and sub-acute lung infections in children will be brought along and used either during this session or as part of discussion of other papers.