The municipality of Leiden has a population of 127,046 (31 January 2023),[7] but the city forms one densely connected agglomeration with its suburbs Oegstgeest, Leiderdorp, Voorschoten and Zoeterwoude with 215,602 inhabitants.

The Netherlands Central Bureau of Statistics (CBS) further includes Katwijk in the agglomeration which makes the total population of the Leiden urban agglomeration 282,207 and in the larger Leiden urban area also Teylingen, Noordwijk, and Noordwijkerhout are included with in total 365,913 inhabitants.

It is a member of the League of European Research Universities and positioned highly in all international academic rankings.

Modern scientific medical research and teaching started in the early 18th century in Leiden with Boerhaave.

[9] In 1420, during the Hook and Cod wars, Duke John III of Bavaria along with his army marched from Gouda in the direction of Leiden in order to conquer the city since Leiden did not pay the new Count of Holland Jacqueline, Countess of Hainaut, his niece and only daughter of Count William VI of Holland.

Burgrave Filips of Wassenaar and the other local noblemen of the Hook faction assumed that the duke would besiege Leiden first and send small units out to conquer the surrounding citadels.

One of Christoffel's pupils was Lodewijk Elzevir (1547–1617), who established the largest bookshop and printing works in Leiden, a business continued by his descendants through 1712.

In 1572, the city sided with the Dutch Revolt against Spanish rule and played an important role in the Eighty Years' War.

It was besieged from May to October 1574 by the Spanish but was relieved by the cutting of the dikes, thus enabling ships to carry provisions to the inhabitants.

[15] It played a crucial role in the establishment of modern chemistry and medicine due to the work by Herman Boerhaave (1668–1738).

[17] On 12 January 1807, a catastrophe struck the city when a boat loaded with 17,400 kg (38,360 lb) of gunpowder blew up in the middle of Leiden.

[18] In 1842, the railroad from Leiden to Haarlem was inaugurated and one year later the railway to The Hague (Den Haag) was completed, resulting in some social and economic improvement.

Johan Rudolf Thorbecke (1798–1872) wrote the Dutch Constitution in April 1848 in his house at Garenmarkt 9 in Leiden.

Leiden's reputation as the "city of books" continued through the 19th century with the establishment of publishing dynasties by Evert Jan Brill and Albertus Willem Sijthoff.

[19] Sijthoff, who rose to prominence in the trade of translated books, wrote a letter in 1899 to Queen Wilhelmina regarding his opposition to becoming a signatory to the Berne Convention for the Protection of Literary and Artistic Works.

Another development was in cryogenics: Heike Kamerlingh Onnes (1913 Nobel Prize in Physics) liquefied helium for the first time (1908) and later managed to reach a temperature of less than one degree above the absolute minimum.

[21] It typically takes place over the course of two to three days and includes parades, a hutspot feast, historical reenactments, a funfair and other events.

The open space for the park was formed by the accidental explosion of a ship loaded with gunpowder in 1807, which destroyed hundreds of houses, including that of the Elsevier family of printers.

A hundred buildings in the centre are decorated with large murals of poetry, part of a wall poem project active from 1992, and still ongoing.

[23][24] At the strategically important junction of the two arms of the Oude Rijn stands the old castle de Burcht, a circular tower built on an earthen mound.

van der Werff) and the Pieterskerk (church of St Peter (1315)) with monuments to Scaliger, Boerhaave and other famous scholars.

This church was given to the Catholics after the gunpowder explosion in 1807, which killed 150 inhabitants and destroyed a large part of the city centre.

Among the institutions connected with the university are the national institution for East Indian languages, ethnology and geography; the botanical gardens, founded in 1587; the observatory (1860); the museum of antiquities (Rijksmuseum van Oudheden); and the ethnographical museum, of which P. F. von Siebold's Japanese collection was the nucleus (Rijksmuseum voor Volkenkunde).

[31] formely Arivva (until 15 dec. 2024) Railway stations within the municipality of Leiden are: The following is a selection of important Leidenaren throughout history: Leiden is twinned with: Nae zWarte HVnger-noot GebraCht had tot de doot bInaest zes-dVIzent MensChen; aLst god den heer Verdroot gaf hI Vns Weder broot zo VeeL WI CVnsten WensChen.

(Dutch: "When the Black Famine had brought to the death nearly six thousand persons, then God the Lord repented, and gave bread again as much as we could wish".