The U.S. Environmental Protection Agency (U.S. EPA) Office of Research and Development (ORD) National Center for Environmental Assessments' (NCEA) mission is to provide guidance and risk assessments aimed at protecting human health and the environment. To accomplish this mission, NCEA works to develop and improve the models, databases, tools, assumptions, extrapolations used in risk assessments. NCEA established the Exposure Factors Program to develop tools and databases that improve the scientific basis of exposure and risk assessment by: (1) identifying exposure factors needs in consultation with clients, and exploring ways for filling data gaps; (2) compiling existing data on exposure factors needed for assessing exposures/risks; and (3) assisting clients in the use of exposure factors data. The Exposure Factors Handbook and the Child-specific Exposure Factors Handbook, as well as other companion documents, such as Example Exposure Scenarios, are products of the Exposure Factors Program. The Exposure Factors Handbook provides information on various physiological and behavioral factors commonly used in assessing exposure to environmental chemicals. The handbook was first published in 1989 and was updated in 1997. Since then, new data have become available. This updated version incorporates data available since 1997 up to June 2009. It also reflects the revisions made to the Child-Specific Exposure Factors Handbook, which was updated and published in 2008. Each chapter in the revised Exposure Factors Handbook presents recommended values for the exposure factors covered in the chapter as well as a discussion of the underlying data used in developing the recommendations. These recommended values are based solely on NCEA's interpretations of the available data. In many situations different values may be appropriate to use in consideration of policy, precedent, or other factors.