A hull is the body or frame of a ship or boat. It is a central concept in water vessels. The hull is essentially what keeps the water from entering the boat and acts as the walls and floor of the vessel.

Nearly all watercraft, from small boats to the largest ships adhere to one general class of hull shapes that serve the needs of stability and efficient propulsion: horizontal cross-sections that have narrow, usually pointed, fronts (at the bow), smooth widening from the bow until roughly the middle (the beam), and often narrowing smoothly but usually significantly to the extreme end (the stern), whose width may range from a large to an insignificant fraction of the beam width. Vertical cross-sections perpendicular to the beam tend to be open on small boats, or flat-decked (with various superstructures) on large boats or on ships; below that level, they may widen to some extent, but almost always narrow smoothly to either a relatively flat bottom or to an angled joint at the center, which may feature a keel or rectractable centerboard. Nevertheless, other general shapes are feasible; the coracle is a relatively extreme example, and many cargo barges, with all cross-sections close to rectangular, are a radical departure from both the coracle and the tapered hulls described above. Large ships have a bulbous bow to increase fuel efficiency.

In hulls constructed from materials that are denser than water, such as steel, the hull traps a volume of air that lowers the overall density of the boat providing buoyancy so that the boat floats. Hulls constructed of materials that are less dense than water, such as some types of wood, may float even when full of water.

The very first hull is thought to have consisted of a hollowed out tree bole and was a Stone Age invention--in effect the first canoe. Hull construction then proceeded to keeled hulls, including ballast and on to modern double steel hulls with waterproof sections.

Hull construction is usually performed in a dry dock or on dry land. In the very latest sailing ships, hulls are often made of layers of foam and plastic, forming composite hulls, with a minimum of weight. Variations on the single hull can be found with outriggers, and craft with more than one hull, called multihulls.