A bridge is a structure designed to span an obstacle, such as a river or valley, allowing vehicles, pedestrians, and other loads to pass across. The Romans and ancient Chinese built major arch bridges of stone and timber. During the Renaissance, advances in science and engineering led to wider bridge spans and more elegant designs. Concrete was perfected in the early 1800s, and proved to be superior to stone in many regards. With the Industrial Revolution came mass-produced steel, which enabled the creation of suspension and cable-stayed bridges that could span wide obstacles. Over time, the maximum achievable span of bridges has steadily increased, reaching 2 kilometers (1.2 miles) in 2022.
Most bridges consist of a flat deck, supported by structures such as beams, arches, or cables. These structures rest on a foundation that is carefully designed to prevent the bridge from settling into the subsoil. Bridges can be constructed in a wide variety of forms, depending on their purpose and location. Notable types include viaducts, which cross wide valleys; trestles to carry heavy trains; and pontoon bridges which float on water.