Asthma is a clinical syndrome that affects all age groups. Asthma prevalence worldwide has seen a rapid increase in the latter part of the last century. Recent data has shown that asthma prevalence has plateaued and even decreased in some areas of the world, despite continuing to increase in other areas of the world. Many risk factors have been associated with asthma and the differences in distributions of these risk factors may explain the differences in prevalence. This article will review recent trends in the prevalence of asthma and recent studies that investigate risk factors of asthma.