The highest court in the land is set to have its say on one of the most important constitutional cases in British legal history.
The Supreme Court is being asked to decide on an earlier ruling that infuriated Brexiteers and led to High Court judges being labelled "Enemies of the People" by the Daily Mail.
Here's everything you need to know.
Who is hearing the case?
Eleven Supreme Court justices - a record number to hear an appeal.
What is the issue?
The panel is being asked by the Government to overturn a High Court ruling that the Prime Minister must seek MPs' approval to trigger the process of taking Britain out of the European Union.
Who made the ruling at the High Court and when?
A panel of three judges in London, led by Lord Chief Justice Lord Thomas, made the ruling on November 3.
Who is presenting the case for the Government?
The Government's top law officer, Attorney General Jeremy Wright, will be arguing the case for quashing the High Court decision.
What will he argue?
The justices will be asked to find that the High Court "erred" in its ruling and that it's for the Government alone to exercise powers in the conduct of the UK's international affairs.
Who else will be presenting arguments?
The justices will hear from a wide range of parties, including the Scottish and Welsh Governments and representatives from Northern Ireland.
How long will the case take?
The legal submissions will be heard over four days.
When will there be a ruling?
Not until the new year.