Nitrogen, oxygen, and argon represent more than 99.9% of the air we breathe. But Earth’s atmosphere hasn’t always had that composition—it is on at least its third distinctive atmosphere. ‘Atmospheric composition’ provides a brief history of Earth’s atmosphere, before considering the two most important regions of the atmosphere for human survival—the stratosphere and troposphere. The stratospheric ozone layer shields harmful ultraviolet-B light penetrating to the surface, thereby protecting humans and ecosystems from harmful ultraviolet radiation. The troposphere is where billions of people live and breathe. It is also where air pollutants are emitted, wildfires burn, vegetation grows, and where the oceans exchange gases. The impact of atmospheric aerosols and greenhouse gases is also discussed.