An oil filter is a filter designed to remove contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil. Their chief use is in internal-combustion engines for motor vehicles, powered aircraft, railway locomotives, ships and boats, and static engines such as generators and pumps. Other vehicle hydraulic systems, such as those in automatic transmissions and power steering, are often equipped with an oil filter. Gas turbine engines, such as those on jet aircraft, also require the use of oil filters. Oil filters are used in many different types of hydraulic machinery. The oil industry itself employs filters for oil production, oil pumping, and oil recycling. Modern engine oil filters tend to be "full-flow" (inline) or "bypass".

Filtration is a physical separation process that separates solid matter and fluid from a mixture using a filter medium that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter medium are described as oversize and the fluid that passes through is called the filtrate. Oversize particles may form a filter cake on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter, known as blinding. The size of the largest particles that can successfully pass through a filter is called the effective pore size of that filter. The separation of solid and fluid is imperfect; solids will be contaminated with some fluid and filtrate will contain fine particles. Filtration occurs both in nature and in engineered systems; there are biological, geological, and industrial forms.