Located about 78 km (48 mi) south of Frankfurt, Heidelberg is part of the densely populated Rhine-Neckar Metropolitan Region which has its center in Mannheim.

[4] Heidelberg is a scientific hub in Germany and home to several internationally renowned research facilities adjacent to its university, including the European Molecular Biology Laboratory and four Max Planck Institutes.

Other parts (mostly quarters from around the 19th century or newer, or originally independent, later incorporated villages) sprawl out into the Rhine Plain and along the Bergstraße (lit.

[8] The following towns and communes border the city of Heidelberg, beginning in the west and in a clockwise direction: Edingen-Neckarhausen, Dossenheim, Schriesheim, Wilhelmsfeld, Schönau, Neckargemünd, Bammental, Gaiberg, Leimen, Sandhausen, Oftersheim, Plankstadt, Eppelheim (all part of the Rhein-Neckar-Kreis), and Mannheim.

The almost year round warm temperatures are usually determined by Atlantic air masses in the winter and increased influence from the western Mediterranean in summer.

In contrast to the nearby Upper Rhine Plain, Heidelberg's position in the valley leads to more frequent easterly winds than average.

Precipitation levels begin to increase and, due to the decreasing sunlight, the region is affected by fog more frequently from the second part of October onwards.

The late Roman Emperor Valentinian I, in 369 AD, built new and maintained older castra (permanent camps) and a signal tower on the bank of the Neckar.

[16] Heidelberg University played a leading part in Medieval Scholasticism, Renaissance humanism, the German Reformation, and in the subsequent conflict between Lutheranism and Calvinism during the 15th and 16th centuries.

One of the aims of the catechism was to counteract the teachings of the Roman Catholic Church as well as Anabaptists and "strict" Gnesio-Lutherans like Tilemann Heshusius and Matthias Flacius, who were resisting Frederick's Reformed influences, particularly on the matter of Eucharist (the Lord's Supper).

In 1810 the French Revolution refugee Count Charles Graimberg began to preserve the palace ruins and establish a historical collection.

Between 1920 and 1933, Heidelberg University became a center of notable physicians Czerny, Erb, and Krehl; and humanists Rohde, Weber, and Gandolf.

[20] In 1934 and 1935 the Reichsarbeitsdienst (State Labor Service) and Heidelberg University students built the huge Thingstätte amphitheatre on the Heiligenberg north of the town, for Nazi Party and SS events.

[23] When his command was ordered to shell the city, Beiderlinden took the initiative to contact the burgomaster and attempt to persuade Nazi soldiers to abandon their positions.

[23] On 29 March 1945, German troops left the city after destroying three arches of the old bridge, Heidelberg's treasured river crossing.

In 1945, the university was reopened relatively quickly on the initiative of a small group of professors, among whom were the anti-Nazi economist Alfred Weber and the philosopher Karl Jaspers.

At the end of December 2011, the city had 149,633 inhabitants with an official primary residence in Heidelberg (not including the soldiers and employees of the U.S. Army and their dependents, a total of about 20,000 people), a historic high.

The most recent city council election was held on 9 June 2024, and the results were as follows: Religion in Heidelberg (2011)[30][31] The "old town" (German: Altstadt), on the south bank of the Neckar, is long and narrow.

It is dominated by the ruins of Heidelberg Castle, 80 metres above the Neckar on the steep wooded slopes of the Königstuhl (King's chair or throne) hill.

In 1720, Karl III Philip, Elector Palatine came into conflict with the town's Protestants as a result of giving the Church of the Holy Spirit exclusively to the Catholics for their use.

Due to pressure by the mostly Protestant powers of Prussia, Holland, and Sweden, Prince Karl III Philip gave way and repartitioned the church for joint use.

Among the prominent thinkers associated with the institution are Georg Wilhelm Friedrich Hegel, Karl Jaspers, Hans-Georg Gadamer, Jürgen Habermas, Karl-Otto Apel, and Hannah Arendt.

This university has many areas of study for national students such as; theology, law, philosophy, modern languages, economics, and social sciences.

The A 5 autobahn runs through the western outskirts of Heidelberg, connecting the region to Frankfurt am Main in the north and Karlsruhe to the south.

Heidelberg Central Station (Hauptbahnhof) is on the Rhine Valley Railway and is served by Intercity-Express, Euro City trains.

On 16 March 1902, the first electric tram ran on Rohrbacher Straße, sharing use of the suburban tracks built by the Deutsche Eisenbahn-Gesellschaft in 1901 between Heidelberg and Wiesloch.

The main military facilities were Campbell Barracks, the former Wehrmacht Grossdeutschland-Kaserne, which housed the US Army, Europe (USAREUR) and NATO's Component Command-Land Headquarters.

These installations, including Tompkins Barracks and Kilbourne Kaserne in nearby Schwetzingen, plus the Germersheim Depot, made up the U.S. Army Garrison Heidelberg.

In March or April the Heidelberger Frühling, the Classic Music Festival and the international Easter egg market are conducted.

The 1925 song "I Lost My Heart in Heidelberg" composed by Fred Raymond was a major hit and inspired a stage musical and two films.