The Russian Zarya, the first module of the International Space Station, was launched on 20 November 1998.
The first American module, Unity, was attached to Zarya three weeks later, followed by the second Russian module, Zvezda, in 2000. The station has been manned continuously since 2000.
Over the past 15 years, five space agencies representing 16 countries have worked together to build the orbiting research centre – one of the most complex scientific and technological endeavours ever undertaken.
NASA astronaut Suni Williams hosted a tour of the station before she returned to Earth earlier this year, shedding light on aspects of life on board such as how the astronauts work, sleep, eat and go to the toilet.
You can also follow the birthday celebrations on Twitter for the next few hours.