There is increasing evidence that the intestinal microbiota impact immune development and disease. Recent evidence suggests that shifts in resident microbiota (normal flora) may play a significant role in atopic diseases such as asthma. Allergic asthma is an ever-increasing problem in developed countries and affects up to 20% of all Canadians. The aim of this project was to explore the contribution of the microbiota to immune development and asthma in both mice and human hosts. They demonstrated that changes in the intestinal microbiota within a critical window of development does impact the development of asthma later in life. Using data collected from a cohort of 319 human subjects from the CHILD Study, the project identified that a significant decrease in four bacterial genera were linked to an increased risk in developing asthma. These results were confirmed using a murine model for asthma and reintroduction of these ‘missing’ microbes in asthmatic mice ameliorated lung inflammation. The identification of key microbes in children at risk for developing asthma may impact the way asthma is diagnosed, treated and potentially prevented in the future. Overall, this project was a success in providing experimental evidence for the role of the gut microbiome in the development of asthma, using both murine and human data.