Automobile inspection stickers are a form of proof for official vehicle safety and emissions certification. Each state has its own rules in regards to inspection, with some requiring a sticker for proof of emissions testing, while others do not require a sticker at all.
The automobile inspection sticker system was designed to provide immediate visual proof of a cars compliance with that state's inspection laws, similar to the way states require a registration sticker on the vehicle's license plate to prove that the vehicle is registered with the Department of Motor Vehicles. As each state is allowed to set its own guidelines, some states do not require a separate sticker for proof of emissions certification. For example, as of March 2015, Texas combined its registration sticker with its emissions inspection sticker, making the inspection a requirement of registration renewal.
Much of the legislation regarding emissions inspections stems from the Clean Air Act, a congressional amendment that requires each state to implement mandatory testing in every major metropolitan city that did not meet minimum federal air quality standards. As a result, states with high populations in major urban areas have strict testing laws compared to less densely populated areas. For example, states like California have annual emissions testing laws.