Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that consist of refined crude oil. A fossil fuel, petroleum is formed when large quantities of dead organisms, mostly zooplankton and algae, are buried underneath sedimentary rock and subjected to both prolonged heat and pressure.

Petroleum is primarily recovered by oil drilling. Drilling is carried out after studies of structural geology, sedimentary basin analysis, and reservoir characterisation. Recent developments in technologies have also led to the exploitation of other unconventional reserves such as oil sands and oil shale.

Once extracted, oil is refined and separated, most easily by distillation, into innumerable products for direct use or use in manufacturing. Products include fuels such as gasoline (petrol), diesel, kerosene and jet fuel; asphalt and lubricants; chemical reagents used to make plastics; solvents, textiles, refrigerants, paint, synthetic rubber, fertilizers, pesticides, pharmaceuticals, and thousands of others. Petroleum is used in manufacturing a vast variety of materials essential for modern life, and it is estimated that the world consumes about 100 million barrels (16 million cubic metres) each day. Petroleum production can be extremely profitable and was critical to global economic development in the 20th century, with some countries, so-called "oil states", gaining significant economic and international power because of their control of oil production.

Petroleum exploitation and use have had significant negative environmental and social consequences. Extraction, refining and burning of petroleum fuels all release large quantities of greenhouse gases, so petroleum is one of the major contributors to climate change. Other negative environmental effects include oil spills, and air and water pollution. Some of these effects have direct and indirect health consequences for humans. Oil has also been a source of conflict, leading to both state-led-wars and other conflicts. Production of petroleum is estimated to reach peak oil before 2035 as global economies lower dependencies on petroleum as part of climate change mitigation and a transition towards renewable energy and electrification.