Intrauterine and early life exposures are important contributors to allergic sensitisation and to the subsequent expression of allergic disease and asthma. The recent epidemic of asthma and atopy clearly underlines the strength of environmental factors such as respiratory and enteric infections, allergen exposure, diet and lifestyles including exposure to cigarette smoke. These and other as yet unidentified environmental contributors are also likely to interact with underlying genetic predisposition. Although recent research interest has focussed on host responses and priming of TH2 lymphocytes there are some paradoxes yet to be resolved. What are the determinants of "homing" of sensitisation to the airway and does persistent airway inflammation result in airway remodelling or are these features independent risk factors for persistence and disease? Evidence that environmental exposures in fetal life and infancy influence lifelong respiratory health remains controversial. However it is becoming apparent that a range of environmental exposures and conditions do influence the development of asthma and atopic disease and that whole population interventions aimed at primary prevention might soon become realistic.