The introduction of 16s ribosomal RNA sequencing as a nonculture technique has led to the discovery of the presence of microbiota in the lower airways of healthy individuals. These bacterial communities may originate from the mouth and nasopharynx or from the environment by inhalation. The microbial composition of the lower airways may be modulated by dietary factors, antibiotic therapy, and microbial infections, particularly in early life. In addition, circulatory products from gut microbiota may influence the lung microbiota to maintain mucosal immunity. Recent studies have revealed that, in asthma, the lower airway microbiota show reduced diversity and community composition that is linked to severity and inflammatory phenotype. There is also a greater prevalence of proteobacteria, including Haemophilus, in symptomatic asthma. Microbial dysbiosis may contribute to both the inception and progression of asthma in infants and children, and to corticosteroid resistance in asthma. A better understanding of the regulation of the lung and gut microbiota in asthma may pave the way for targeting microbiota to prevent and treat asthma.