The Berlin U-Bahn (commonly understood to be short for Untergrundbahn - "underground railway") is a network of ten lines across the city. They are numbered from 1 to 9 with the prefix "U" ( U1  U2  U3  U4  U5  U6  U7  U8  U9 ), with the additional line U55 ( U55 ) in operation until its route gets connected to the U5 sometime 2020. You may find the U-Bahn network slightly less logical and convenient to use than in other European capitals, as Berlin's troubled history made its mark on it and many key locations remain unconnected, which is why using buses, trams and S-Bahn to complement the U-Bahn is probably necessary for efficient travel throughout Berlin. However as those systems are fully integrated (see above), you can do so with only one ticket or type of ticket. Generally speaking in the east trams are more widespread while the west relies more heavily on U-Bahn, but that has been slowly changing since 1990.

Despite the name "underground", some 20% of the network is actually made up of overground stretches running over characteristic viaducts throughout the city, adding a certain flavour to Berlin's cityscape. This arrangement is similar to many older subway systems which include elevated or even at-grade sections like the Hamburg system or the M2 / M6 lines in Paris. Unlike light rail systems or the Berlin tram however, all parts of the network have their own right of way and subways don't have level crossings.

Detailed maps can be found in every U-Bahn station and on the trains. U-Bahn stations can be seen from far by their big, friendly blue U signs. Together with the S-Bahn (which is administered by Deutsche Bahn and mostly runs aboveground), the U-Bahn provides a transportation network throughout greater Berlin that is extremely efficient and fast. On weekends (Friday to Sunday), and during the Christmas and New Year holidays, all U-Bahn and S-Bahn lines (except line U4 and U55) run all night, so returning from late night outings is easy, especially given the average start time of most 'parties' in Berlin (23:00 to 01:00). During the week there is no U-Bahn or S-Bahn service from c. 01:00-04:30, but metro trams/buses and special Night Buses (parallel to the U-Bahn line) run every half an hour 12:30-04:30.

There are no turnstiles to limit access to U-Bahn stations: it is technically possible to ride without a ticket. But if caught by a ticket checker you will be fined €60 (see "Validating tickets" above) so it is not only illegal but probably not worth the risk to ride without validating your ticket. Nearly all U- and S-Bahn stations now have electronic signs that display the expected arrival of the next train (and its direction), based on sensors along the lines.

Design-wise, U-Bahn stations are about as diverse as you'd expect for a system that started operations when the Kaiser reigned and has been expanded in phases of overflowing as well as empty municipal coffers. Quite a handful of stations built before World War II were designed by the Swedish architect Alfred Grenander (died 1931) whose designs are much lauded and who included some useful features like having each station dominated by a certain color which - where it is still visible to the untrained eye - helps in quickly recognizing a station. From the mid 1960s to mid 1990s most stations built in West Berlin were designed by Rainer G. Rümmler (1929-2004) who gave each station a much more individual look compared to Grenander who preferred to vary only small details like the color of the tiles. There are of course other stations, designed by other architects and for the new U5 extension station "Museumsinsel" a rather grandiose design has been chosen that will surely impress visitors when the station opens.