The incidence, prevalence, and mortality of asthma have increased in children over the past three to four decades, although there has been some decline in the most recent decade. These trends are particularly marked and of greatest concern in preschool children. Internationally, there are huge variations among countries and continents, as demonstrated by the International Study of Asthma and Allergies in Childhood. In general, asthma rates were highest in English-speaking countries (UK, New Zealand, Australia, and North America) and some Latin American countries (Peru and Costa Rica), and lowest in South Korea, Russia, Uzbekistan, Indonesia, and Albania. There is currently no unifying hypothesis to explain these trends or any associated risk factors. Environmental factors that may lead to asthma include air pollution; genetic factors, the hygiene hypothesis, and lifestyle differences also play potentially causative roles. Asthma may develop as a result of persistent activation of the immune system alone or in combination with physiologic airway remodeling in early childhood. Further studies are needed to confirm this hypothesis.