The tree was chopped down in Nordmarka (The North Soil) Forest on the outskirts of Oslo. The exact tree was chosen because it is tall, lush and green. The branches cover the whole length of the trunk and it's located near enough to a road to get a crane near it.
Once cut, the tree had to be lowered onto the back of an open lorry and securely tied down so no branches would break in transit. That lorry was then escorted at night by police along closed off roads from Oslo to the port of Brevik.
At the port the "Queen of the Forest" was loaded onto a ship before starting a 550 mile journey across the North Sea to England.
It arrived at the port of Immingham near Grimsby and was stored in a secret location. The exact route it took to London is also a closely guarded secret for security reasons.
At 20 metres (60ft) tall, it's twice the length of a double decker bus. That can be awkward winding through London's ancient streets so the journey was made in the early hours well before rush hour.
It arrived at Trafalgar Square at 6am this morning. A specialist rigging team erected the tree using a hydraulic crane. It has been decorated in the traditional Norwegian fashion, with vertical strings of white lights.
What's next?
The tree's 900 LED lights will be switched on by Boris Johnson on Thursday night.
The tree will remain in Trafalgar Square until just before the Twelfth Night of Christmas, when it will be taken down for recycling. It will then be chipped and composted, to make mulch.
