A crankcase ventilation system removes unwanted gases from the crankcase of an internal combustion engine. The system usually consists of a tube, a one-way valve and a vacuum source (such as the inlet manifold).

The unwanted gases, called "blow-by", are gases from the combustion chamber which have leaked past the piston rings. Early engines released these gases to the atmosphere simply by them leaking through the crankcase seals. The first specific crankcase ventilation system was the 'road draught tube', which used a partial vacuum to draw the gases through a tube and release them to the atmosphere. Positive crankcase ventilation (PCV) systems— first used in the Second World War and present on most modern engines— send the crankcase gases back to the combustion chamber, as part of the vehicle emissions control, in order to reduce air pollution.

Two-stroke engines with a crankcase compression design do not need a crankcase ventilation system, because normal operation of the engine involves sending the crankcase gases to the combustion chamber.