This chapter describes how air is a complex natural gaseous system essential to support life on Earth. Air pollution comes from a wide variety of sources, which discharge of harmful substances into the atmosphere, causing adverse effects to humans and the environment. They can be natural or anthropogenic. Natural air pollution sources are multiple and include volcanic eruption, fire, ocean vapors, dust storms and fermentation of organic materials. However, the range and quantities of chemicals discharged into the atmosphere from industry, transport, agriculture, energy production, domestic heating, and many other human activities, have increased dramatically. Some pollutants are emitted directly into the atmosphere and are known as primary pollutants (NOx, SOx, particulate matter, etc.). Others are formed in the air as a result of chemical reactions with other pollutants and atmospheric gases; these are known as secondary pollutants like ozone. This chapter provides an overview on air pollution sources as well as the ways in which pollutants can affect human health and the environment.