Tunnel warfare refers to aspects of warfare relating to tunnels and other underground cavities.

It includes the construction of underground facilities in order to attack or defend, and the use of existing natural caves and artificial underground facilities for military purposes. Tunnels can be used to undermine fortifications and slip into enemy territory for a surprise attack, while it can strengthen a defense by creating the possibility of ambush, counterattack and the ability to transfer troops from one portion of the battleground to another unseen and protected. Tunnels can serve as shelter from enemy attack.

Since antiquity, sappers have used mining against walled cites, fortresses, castles or other strongly held and fortified military positions. Defenders have dug counter-mines to attack miners or destroy a mine threatening their fortifications. Since tunnels are commonplace in urban areas, tunnel warfare is often a feature, though usually a minor one, of urban warfare. A good example of this was seen in the Syrian Civil War in Aleppo, where in March 2015 rebels planted a large amount of explosives under the Syrian Air Force Intelligence Directorate headquarters.

Tunnels are narrow and restrict fields of fire; thus, troops in a tunnel usually have only a few areas exposed to fire or sight at any one time. They can be part of an extensive labyrinth and have cul-de-sacs and reduced lighting, typically creating a closed-in night combat environment.^{[citation needed]}

The Greek historian Polybius, in his Histories, gives a graphic account of mining and counter mining at the Roman siege of Ambracia:

The Aetolians ... offered a gallant resistance to the assault of the siege artillery and [the Romans], therefore, in despair had recourse to mines and tunnels. Having safely secured the central one of their three works, and carefully concealed the shaft with wattle screens, they erected in front of it a covered walk or stoa about two hundred feet long, parallel with the wall; and beginning digging from that, they carried it on unceasingly day and night, working in relays. For a considerable number of days the besieged did not discover them carrying the earth away through the shaft; but when the heap of earth thus brought out became too high to be concealed from those inside the city, the commanders of the besieged garrison set to work vigorously digging a trench inside, parallel to the wall and to the stoa which faced the towers. When the trench was made to the required depth, they next placed in a row along the side of the trench nearest the wall a number of brazen vessels made very thin; and, as they walked along the bottom of the trench past these, they listened for the noise of the digging outside. Having marked the spot indicated by any of these brazen vessels, which were extraordinarily sensitive and vibrated to the sound outside, they began digging from within, at right angles to the trench, another tunnel leading under the wall, so calculated as to exactly hit the enemy's tunnel. This was soon accomplished, for the Romans had not only brought their mine up to the wall, but had under-pinned a considerable length of it on either side of their mine; and thus the two parties found themselves face to face.

The Aetolians then countered the Roman mine with smoke from burning feathers with charcoal, in essence an early form of chemical warfare.

Another extraordinary use of siege-mining in ancient Greece was during Philip V of Macedon's siege of the little town of Prinassos, according to Polybius, "the ground around the town were extremely rocky and hard, making any siege-mining virtually impossible. However, Philip ordered his soldiers during the cover of night collect earth from elsewhere and throw it all down at the fake tunnel's entrance, making it look like the Macedonians were almost finished completing the tunnels. Eventually, when Philip V announced that large parts of the town-walls were undermined, the citizens surrendered without delay."