The Karl Theodor Bridge, commonly known as the Old Bridge, is an arch bridge in Heidelberg that crosses the Neckar river. It connects the Old City with the eastern part of the Neuenheim district of the city on the opposite bank. The current bridge, made of Neckar sandstone and the ninth built on the site, was constructed in 1788 by Elector Charles Theodore, and is one of the best-known landmarks and tourist destinations in Heidelberg.

The Neckar is a 362-kilometre-long (225 mi) river in Germany, mainly flowing through the southwestern state of Baden-Württemberg, with a short section through Hesse. The Neckar is a major right tributary of the Rhine. Rising in the Schwarzwald-Baar-Kreis near Schwenningen in the Schwenninger Moos conservation area at a height of 706 m (2,316 ft) above sea level, it passes through Rottweil, Rottenburg am Neckar, Kilchberg, Tübingen, Wernau, Nürtingen, Plochingen, Esslingen, Stuttgart, Ludwigsburg, Marbach, Heilbronn and Heidelberg, before discharging on average 145 m3/s (5,100 cu ft/s) of water into the Rhine at Mannheim, at 95 m (312 ft) above sea level, making the Neckar its 4th largest tributary, and the 10th largest river in Germany. Since 1968, the Neckar has been navigable for cargo ships via 27 locks for about 200 kilometres (120 mi) upstream from Mannheim to the river port of Plochingen, at the confluence with the Fils.