thumb|right|upright|Demolition of the Old Myer Building, Perth, Western Australia

Demolition is the tearing-down of buildings and other structures, the opposite of construction. Demolition contrasts with deconstruction, which involves taking a building apart while carefully preserving valuable elements for re-use.

For small buildings, such as houses, that are only two or three stories high, demolition is a rather simple process. The building is pulled down either manually or mechanically using large hydraulic equipment: elevated work platforms, cranes, excavators or bulldozers. Larger buildings may require the use of a wrecking ball, a heavy weight on a cable that is swung by a crane into the side of the buildings. Wrecking balls are especially effective against masonry, but are less easily controlled and often less efficient than other methods. Newer methods may use rotational hydraulic shears and silenced rock-breakers attached to excavators to cut or break through wood, steel, and concrete. The use of shears is especially common when flame cutting would be dangerous.

The tallest demolished building was the 47-story Singer Building in New York City, which was built in 1908 and torn down in 1967-1968 to be replaced by One Liberty Plaza.

Building implosion

Demolition of a chimney at the former brewery "Henninger" in Frankfurt am Main, Germany, on 2 December 2006 thumb|right|The demolition of the New Haven Coliseum in New Haven, Connecticut Large buildings, tall chimneys, smokestacks, and increasingly some smaller structures may be destroyed by building implosion using explosives. Imploding a building is very fast — the collapse itself only takes seconds — and an expert can ensure that the building falls into its own footprint, so as not to damage neighboring structures. This is essential for tall structures in dense urban areas.

Any error can be disastrous, however, and some demolitions have failed, severely damaging neighboring structures. The greatest danger is from flying debris which, when improperly prepared for, can kill onlookers.

Even more dangerous is the partial failure of an attempted implosion. When a building fails to collapse completely the structure may be unstable, tilting at a dangerous angle, and filled with un-detonated but still primed explosives, making it difficult for workers to approach safely.

A third danger comes from air overpressure that occurs during the implosion. If the sky is clear, the shockwave, a wave of energy and sound, travels upwards and disperses, but if cloud coverage is low, the shockwave can travel outwards, breaking windows or causing other damage to surrounding buildings.

Stephanie Kegley of CST Environmental described shockwaves by saying, "The shockwave is like a water hose. If you put your hand in front of the water as it comes out, it fans to all sides. When cloud coverage is below 1,200 feet, it reacts like the hand in front of the hose. The wave from the shock fans out instead of up toward the sky."