Infants are a unique subpopulation with regard to aerosol therapy. There are various anatomical, physiological and emotional factors peculiar to infants that present significant difficulties and challenges for aerosol delivery. Most studies about the factors determining lung deposition of therapeutic aerosols are based on data from adults or older children which cannot simply be extrapolated directly to infants. The present review describes why infants/toddlers are very different with respect to two major issues--namely their anatomy/physiology and their behaviour. We suggest possible solutions and future research directions aimed at improving clinical outcomes in this age group.