Through air inhalation, dust ingestion and dermal exposure, the indoor environment plays an important role in controlling human chemical exposure. Indoor emissions and chemistry can also have direct impacts on the quality of outdoor air. And so, it is important to have a strong fundamental knowledge of the chemical processes that occur in indoor environments. This review article summarizes our understanding of the indoor chemistry field. Using a molecular perspective, it addresses primarily the new advances that have occurred in the past decade or so and upon developments in our understanding of multiphase partitioning and reactions. A primary goal of the article is to contrast indoor chemistry to that which occurs outdoors, which we know to be a strongly gas-phase, oxidant-driven system in which substantial oxidative aging of gases and aerosol particles occurs. By contrast, indoor environments are dark, gas-phase oxidant concentrations are relatively low, and due to air exchange, only short times are available for reactive processing of gaseous and particle constituents. However, important gas-surface partitioning and reactive multiphase chemistry occur in the large surface reservoirs that prevail in all indoor environments. These interactions not only play a crucial role in controlling the composition of indoor surfaces but also the surrounding gases and aerosol particles, thus affecting human chemical exposure. There are rich research opportunities available if the advanced measurement and modeling tools of the outdoor atmospheric chemistry community continue to be brought indoors.