A bridge is a construction that crosses a physical barrier without obstructing passage beneath it, such as a body of water, a valley, or a road. It is designed to facilitate passage over a barrier that would otherwise be difficult or impossible to overcome. There are several designs to choose from, each of which serves a certain purpose and may be utilized in a number of situations. The function of a bridge, the nature of the terrain on which it is built and anchored, the material used to construct it, and the amount of money available to construct it all have an impact on the design.

The first bridges were most likely made of fallen trees and stepping stones, while Neolithic people built boardwalk bridges across wetlands. The Arkadiko Bridge in Peloponnese, southern Greece, is one of the oldest arch bridges still in use, dating from the 13th century BC.

The simplest and earliest kind of bridges were stepping stones. The Neolithic humans built a sort of boardwalk over marshes at the Sweet Track and the Post Track in England, both approximately 6000 years old. Ancient humans definitely used log bridges, which are timber bridges that fall naturally or are intentionally felled or placed over streams. Some of the first man-made bridges with significant spans were likely built with trees cut.

One of the earliest timber bridges is the Holzbrücke Rapperswil-Hurden, which spans upper Lake Zürich in Switzerland; prehistoric timber piles discovered west of the Seedamm date back to 1523 BC. The earliest wooden footbridge across Lake Zürich was built in the late 2nd century AD, and it was repaired several times until the Roman Empire built a 6-meter-wide (20-foot) wooden bridge. Between 1358 and 1360, Rudolf IV, Duke of Austria, built a ‘new’ wooden bridge over the lake, which was used until 1878, spanning about 1,450 meters (4,760 feet) in length and 4 meters (13 feet) in width. On April 6, 2001, the restored wooden footbridge, Switzerland’s longest wooden bridge, was opened.

The Arkadiko Bridge is one of four Mycenaean corbel arch bridges that connect the fort of Tiryns with the town of Epidauros in southern Greece as part of an ancient network of chariot routes. It dates from the Greek Bronze Age and is one of the earliest arch bridges currently in use (13th century BC). A number of well-preserved Hellenistic arched stone bridges may be seen in the Peloponnese.

The finest bridge builders of all time were the ancient Romans. Aqueducts and arch bridges built by the Romans could endure conditions that would have destroyed or ruined earlier designs. Some of them have survived to this day. One example is the Alcántara Bridge in Spain, which spans the Tagus River. The Romans also used cement, which reduced the strength fluctuations seen in natural stone. Pozzolana is a kind of cement produced from water, lime, sand, and volcanic rock. When the technique for cement was lost after the Roman era, brick and mortar bridges were built (then later rediscovered).

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