Humans spend the largest percentage of their time in indoor environments, approximately 90%. Therefore, it is critical to characterize exposures to air pollution in indoor environments in order to conduct risk and health impact assessments. Not all indoor environments are the same, hence their contributions to personal exposure will differ. This chapter discusses different factors that might affect exposure to air pollution indoors, including time spent indoors, the influence of outdoor air and the presence of indoor sources of pollution. These factors are also influenced by human, societal and environmental characteristics, e.g. gender, age, employment status and lifestyle. Geopolitics will also influence the contribution of indoor exposures to total exposure, since the economic development of countries affects housing stock, access to energy, employment opportunities and transport infrastructure, all relevant to exposure. Regional climatological conditions will also affect indoor and total exposure to air pollution. This chapter describes the methodology for determining the contribution of indoor environments to total exposure and doses of air pollution. Factors relevant to exposure in indoor environments are discussed, including exposures in developing countries and for sensitive populations.