Berlin is the capital of Germany, as well as its largest city by both area and population. With 3.7 million inhabitants, it has the highest population within its city limits of any city in the European Union. The city is also one of the states of Germany, being the third-smallest state in the country by area. Berlin is surrounded by the state of Brandenburg, and Brandenburg's capital Potsdam is nearby. The urban area of Berlin has a population of over 4.6 million, making it the most populous in Germany. The Berlin-Brandenburg capital region has around 6.2 million inhabitants and is Germany's second-largest metropolitan region after the Rhine-Ruhr region, as well as the fifth-biggest metropolitan region by GDP in the European Union.
Berlin was built along the banks of the Spree river, which flows into the Havel in the western borough of Spandau. The city includes lakes in the western and southeastern boroughs, the largest of which is Müggelsee. About one-third of the city's area is composed of forests, parks and gardens, rivers, canals, and lakes.