A Stacker is a large machine used in bulk material handling. Its function is to pile bulk material such as limestone, ores and Coal on to a stockpile.

Stackers are nominally rated for capacity in tons per hour (T/H). They normally travel on a rail between stockpiles in the stockyard. A stacker can usually move in at least two directions: horizontally along the rail and vertically by luffing (raising and lowering) its boom. Luffing of the boom minimizes dust by reducing the distance that material such as coal needs to fall to the top of the stockpile. The boom is luffed upwards as the height of the stockpile increases. Some stackers can rotate the boom. This allows a single stacker to form two stockpiles, one on either side of the conveyor.