“[We] are basically an indoor species. In a modern society, total time outdoors is the most insignificant part of the day….” (Ott 1989) “We shape our buildings; thereafter they shape us.” (Churchill 1943) Although humans in industrialized countries spend nearly 90% of their time in enclosed buildings (Klepeis et al. 2001), we know very little about the biology of the indoor environment. However this is starting to change. Over the past few years the field of indoor ecology has grown dramatically. Ecologists are beginning to apply ecological theory and concepts to understanding buildings as ecosystems. A new understanding of the biodiversity of built environments is emerging, as well as a new appreciation of the importance of interactions between humans and non-human life indoors. This organized oral session will showcase this emerging understanding. We will feature presentations that demonstrate the utility of ecological theory for understanding built environments, that describe the dynamics of biodiversity indoors and that illustrate the interactions of humans with indoor ecology. Our focus will be on the ecology of the dominant forms of non-human life indoors –microorganisms – and their interactions with humans.