The prevalence of allergic disease has dramatically increased over the past 30 years in Westernised countries. It is unlikely that the rapid increase in the prevalence of allergic disease is the result of genetic changes, which highlights the importance of environmental factors in the development of allergic disease. The ‘hygiene hypothesis’ was put forward in 1989 and focused attention on the notion that exposure to microbes and their products in early life can modify the risk for development of allergic disease. Infections were thought to polarize the immunological response towards a Th2-mediated immune responses causing allergic disease. However it is likely that the Th1/Th2 imbalance is too simplistic to explain the increased prevalence of allergic disease. Current research is focusing on understanding the role of T regulatory cells in inducing a state of tolerance and the resulting modified Th2 response observed in natural and induced tolerance.