Twenty-five years ago, the US Environmental Protection Agency (a US agency), working with Harvard University, was studying the sources of various environmental pollutants. They made what was to them a startling observation: regardless of which community they studied – its location; whether rural or urban; lightly or highly industrialized; and regardless of sex, age, smoking habits, and occupation – indoor air pollution was the major source of exposure to many air pollutants. This is perhaps not surprising: most people spend 90% or more of their time indoors, indoor sources emit many of the same pollutants as outdoors sources, and mixing with outdoor air may be slow to occur. In the following years, the Advisory Board of the US Environmental Protection Agency (a US agency) ranked indoor air pollution as a priority environmental health risk. Section I of this chapter reviews specific contaminants that affect indoor air quality: combustion pollutants (including tobacco smoke), volatile organic chemicals (VOCs), radon, and biological pollutants. It briefly overviews too the major impact that combustion particulates have on people in impoverished homes of less-developed countries. Section II delves into hazardous household products and household hazardous waste, and describes two old hazards that remain with us, asbestos and lead paint.