A drydock is a structure used in the construction, maintenance, and repair of ships and boats. It is built in such a way that it can be filled with water to allow a vessel to sail in, and then drained, leaving the entire body of the vessel exposed so that it can be worked on. Most ship yards have at least one drydock, and some drydocks are extremely large, to accommodate massive ships in need of repair work. A skilled team of ship builders and repair people work on the vessel while it is drydocked, and then flood the drydock so that it can return to the open water and active use.

There are two common types of drydock. The first has been used since antiquity, with writing from both Ancient Greece and Egypt suggesting that drydocks of this type were in active use. The drydock is connected to a harbor or body of water with the use of gates or caissons to separate the main body of water from the drydock. The earth in the area is dug out and supported so that it will not collapse, and the bottom is lined. The drydock is flooded to allow a ship to sail in, and then drained with the use of pumps.

The second type of drydock, a floating drydock, has also been used for hundreds of years in various forms. A floating drydock has a series of chambers which can be filled with water to sink it, or drained and filled with air to lift the drydock, along with a ship which has been positioned inside. Floating drydocks can be used in the open ocean for emergency repairs, and also sailed to locations around the world, making them highly flexible and useful repair tools.

Positioning of a ship in drydock is crucial. Blocks must be aligned along the bottom, or keel, of the ship to support it, and blocks are placed along the sides to prevent the ship from listing in one direction or another. The drydock crew works with blueprints to ensure that the weight of the ship will be supported without causing undue stress to the hull, and divers are often used to perfectly position the ship while the drydock is being drained. Once the ship has been positioned and the drydock drained, work can begin. Some drydocks are also covered, to allow for work in all weathers, and to create a secure area to work on confidential ships, such as submarines.