The purpose of this chapter is to understand what the concept of air pollution means with respect to the ambient air that breathe. This understanding begins with the definition of ambient air. It is followed by providing an understanding of the role that sources of energy play in producing primary, secondary pollutants and greenhouse gases; illustrating that air pollution results from both anthropogenic and natural sources/processes; describing the major types and sources of natural air pollution; and providing an understanding of the differences between smog and haze and how they are formed. This chapter then focuses on the different sources of anthropogenic air pollution. It provides familiarity with national emission estimates of anthropogenic pollutants and their significance; provides familiarity with concentrations, atmospheric processes, and sinks for carbon oxides, sulfur oxides, nitrogen oxides, hydrocarbons, methane, and halogenated hydrocarbons; understanding of atmospheric photochemical processes that result in the productions of O3 and other photochemical oxidants; and understanding of the differences in atmospheric chemistry responsible for elevated ozone levels in urban and rural areas. This chapter also provides an introduction to particulate matter that may be present in ambient air, including familiarity with the size characteristics of particles and particle-forming processes; an understanding of the major chemical components of atmospheric particulate mass; understanding of the factors that affect particle behavior in the atmosphere; a general knowledge of particle concentrations in different areas of the United States; and awareness of long-range particle transport phenomena.