Built environments are the structures that humans create to shelter from the outdoors and provide spaces for living, working, playing and getting places. Along with humans, pets, pests and house plants, built environments house a range of microbes. Preliminary studies indicate that indoor spaces have distinct microbial communities, influenced by building materials, ventilation and airflow, moisture and human and animal activity. The Academy convened a colloquium in Sept. 2015 to examine the role of complex microbial ecosystems found in built environments, including their effects on building chemistry and human health. Studying the microbiology of built environments can change the ways we design, build, operate, occupy and clean our indoor spaces.