Despite extensive use of aerosol therapy to treat infants and young children with respiratory disease, our knowledge of factors influencing drug delivery in this age group remains relatively rudimentary. Recent work with filters used in conjunction with pumps or patients have emphasised some of the factors that will maximise the dose inhaled using different devices though results obtained particularly when used with patients should be interpreted with caution and in context. There are few pharmacokinetic or radiolabelled deposition studies on which to base statements regarding dose likely to reach the lungs of children in this age group. Lung function and clinical results suggest that drugs can be delivered via nebulisers and holding chambers with face masks and inevitably performance of such devices will vary. However, factors such as screaming and non-compliance with treatment are likely to influence the lung dose to a great extent. Hence choice of drug delivery system must be based on patient/parent acceptability as much as on theoretical grounds. Aerosol therapy in this age group is further complicated by our lack of knowledge related to the aetiology of recurrent respiratory symptoms in young children and hence it is quite likely that many children are being treated with effective delivery systems but inappropriate therapeutic agents. Much work is still required before we have a clear understanding of the aetiology and pathology of the distinct sub groups of respiratory disease in young children. Until we have a greater understanding in this area together with improved understanding of delivery systems, drug therapy in this age group will remain very much an empirical art.