Compressed natural gas (or simply shortened as CNG) is a fuel gas mainly composed of methane (CH₄), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of 20–25 megapascals (2,900–3,600 psi; 200–250 bar), usually in cylindrical or spherical shapes.

CNG is used in traditional petrol/internal combustion engine vehicles that have been modified, or in vehicles specifically manufactured for CNG use: either alone (dedicated), with a segregated liquid fuel system to extend range (dual fuel), or in conjunction with another fuel (bi-fuel). It can be used in place of petrol, diesel fuel, and liquefied petroleum gas (LPG). CNG combustion produces fewer undesirable gases than the aforementioned fuels. In comparison to other fuels, natural gas poses less of a threat in the event of a spill, because it is lighter than air and disperses quickly when released. Biomethane, biogas from anaerobic digestion or landfill, can be used.

In response to high fuel prices and environmental concerns, CNG has been used in auto rickshaws, pickup trucks, transit and school buses, and trains.

The cost and placement of fuel storage containers is the major barrier to wider/quicker adoption of CNG as a fuel. It is also why 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 (30 percent per year). Now, as a result of the industry's steady growth, the cost of such fuel storage cylinders has been brought down to a much more acceptable level. Especially, for the CNG Type 1 and Type 2 cylinders, many countries are able to make reliable and cost effective cylinders for conversion need.

CNG's energy density is the same as liquefied natural gas at 53.6 MJ/kg. Its volumetric energy density, 9 MJ/L, is 42 % of that of LNG (22 MJ/L) because it is not liquefied, and is 25 percent that of diesel fuel.

Gases provided the original fuel for internal combustion engines. The first experiments with compressed gases took place in France in the 1850s. Natural gas first became a transport fuel during World War I. In the 1960s, Columbia Natural Gas of Ohio tested a CNG carrier. The ship was to carry compressed natural gas in vertical pressure bottles; however, this design failed because of the high cost of the pressure vessels. Since then, there have been attempts at developing a commercially viable CNG carrier. Several competing CNG ocean transport designs have evolved. Each design proposes a unique approach to optimizing gas transport, while using as much off-the-shelf technology as possible, to keep costs competitive.

In the early 2000s, Canadian company SeaNG developed the Coselle system, a novel CNG carrier design that used coiled steel pipe cartridges to store compressed natural gas more efficiently. This approach received classification approval and demonstrated a potential alternative to traditional vertical pressure vessels, which had been previously limited by high costs and weight.

Around the same time, major classification societies such as Det Norske Veritas (DNV) and the American Bureau of Shipping (ABS) introduced formal certification rules for CNG carriers. These standards helped establish safety and design criteria that allowed CNG vessel projects to progress toward commercial viability, promoting broader acceptance within the maritime industry.