The aim of this paper is to outline, in the light of present understanding of the disease, some of the likely causes for the increasing prevalence and mortality associated with asthma in different communities in the world. It seems important both to understand the causes of asthma and to taking action to reverse the trends. There is an increasing sense of urgency in addressing the problems of morbidity and mortality caused by asthma. In this regard the respiratory community is a long way behind the cardiovascular community who have been documenting the risk factors for vascular disease for many years. In the absence of a reliable definition of asthma, it is difficult to make definite statements about trends in its prevalence, morbidity and mortality. Nevertheless, as already summarised, asthma appears to be an increasing problem. the trend towards increasing deaths in some countries is particularly worrying. The mortality data are most accurate for the 5 to 34 year olds and this paper is confined to reviewing data from this age group.