A head gasket is something used in an internal combustion engine to create a seal between the head(s) (containing the valves and (usually) the camshafts) and the engine block (containing the pistons and the crankshaft). The head gasket performs several functions. It aids in creating a close to near-perfect seal in the combustion chamber formed in the space where the head and cylinders meet, so that there is no leakage from the engine in the combustion process. The gasket also acts to seal the interconnecting coolant passages that flow through the head and the block, so that none of the coolant will leak into the combustion chamber or the top of the head where the valves or cams are.

Head gaskets are prone to rupture (or "blowing") due to excessive combustion chamber pressure (i.e. from aftermarket turbocharging systems), or from extreme engine temperature (coolant becoming too hot and boiling, sometimes explosively).