Ambient air pollution is the foremost reason for global death and disease. An estimated premature death globally is related to ambient air pollution, mainly from emphysema, obstructive bronchiolitis, lung cancer, heart disease, stroke, and severe respiratory problems in children. The criteria air pollutants include particulate matter (PM), ozone (O-3), nitrogen dioxide (NO2), sulphur dioxide (SO2) and lead (Pb). The present chapter provides a summary of the types of criteria air pollutants, their National Ambient Air Quality Standards and their emission sources. This chapter also explains their level distribution and chemistry, and the sink in the earth's environment of these criteria pollutants is studied extensively. Description of global, regional emissions of criteria air pollutants, their contribution from different sectors, and efficiency of control strategies in developed and developing countries are also focused.