Compressed natural gas (CNG or Compressed Natural Gas) (methane stored at high pressure) is a fuel that can be used in place of gasoline (petrol), Diesel fuel, and propane/LPG. CNG combustion produces fewer undesirable gases than the fuels mentioned above. It is safer than other fuels in the event of a spill because natural gas is lighter than air and disperses quickly when released. CNG may be found above oil deposits or may be collected from landfills or wastewater treatment plants where it is known as biogas.

CNG is made by compressing natural gas (which is mainly composed of methane, CH4), to less than 1 percent of the volume, it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of 20-25 MPa (2,900-3,600 psi), usually in cylindrical or spherical shapes.

CNG is used in traditional gasoline/internal combustion engine automobiles that have been modified or in vehicles that were manufactured for CNG use. Natural gas vehicles are increasingly used in the Indian capital of Delhi, and other large cities like Ahmedabad, Mumbai, Pune, Kolkata as well as cities such as Lucknow, Kanpur, etc. CNG is starting to be used also in tuk-tuks and pickup trucks, transit and school buses, and trains.

The cost and placement of fuel storage tanks is the major barrier to wider/quicker adoption of CNG as a fuel. It is also why the municipal government, public transportation vehicles were the most visible early adopters of it, as they can more quickly amortize the money invested in the new (and usually cheaper) fuel. In spite of these circumstances, the number of vehicles in the world using CNG has grown steadily.