Some lines will now run for 24 hours.
Victoria, Central, Northern, Jubilee and Piccadilly lines will offer that round the clock the access. That's the "initial" network, so there could be more to come. Here's how they look on the map:
The service is intended to complement night bus operations, "giving passengers an extensive and integrated service throughout all hours of Friday and Saturday nights."
From 2015, the new round the clock service will be fully staffed and controlled while services are operating. Here's London mayor Boris Johnson showing off the map.
Johnson says that the service is "very important in a 24 hour city".