A smog check is a test done on the exhaust system of a motor vehicle to determine how many pollutants, and of which type, the vehicle is emitting. The smog check also includes an assessment of other parts of the vehicle to make sure that they are in working order, especially those parts which pertain to emissions control. In the United States, most states have smog check requirements which vary from state to state in a nationwide effort to address clean air issues.

Some states have a biannual smog check program, while others require testing every year. Most states exempt certain vehicles from the smog check, such as vehicles more than thirty or less than five years old. Most states require a successful smog check to sell a vehicle, and will require a smog certificate before the title can be transferred. Consumers should check with their state's Department of Motor Vehicles to find out more about the smog requirements for their area.

All states have emissions limits which vehicles cannot exceed. During a smog check, a device is connected to the tailpipe while the engine is run to determine the emissions of the car in idle and while being revved, which are measured against the state standard for that type of vehicle. The smog technician also checks to make sure that the muffler is in working order, and usually opens the hood to make sure that all hoses and pipes are properly connected, and that the car's internal computer is working properly.

Emissions loads which exceed the state limit, an improperly maintained muffler and exhaust system, or the presence of a “check engine” light can cause a vehicle to fail the smog check. In this instance, the vehicle must be repaired and tested again. Most repairs to pass a smog test are relatively simple and do not involve great expense. They are well worth it to ensure that the vehicle is safe and legal. In some cases, the repair may represent a substantial expense. Some states have smog abatement programs, in which drivers who can prove that the car requires repairs over a certain dollar amount can be exempted from smog requirements.

Drivers should get a smog check for their vehicles regularly, not only because it is legally required in most areas, but because if their vehicle is not in alignment with emissions standards, it should be repaired. Personal vehicles contribute almost 40% of pollution in some states, and responsible drivers should aid the national effort to reduce emissions. In addition, some factors which may cause a vehicle to fail a smog check are also unsafe, and should be addressed to prevent accident or injury.