The man who claims he was the mastermind behind the September 11 terror attacks in 2001 will appear in court with four others after years of legal wrangling to hear charges against him.
In what has been called the trial of the century, Khalid Sheikh Mohammed and four alleged co-conspirators face a total of seven charges arising from the attacks which killed almost 3,000 more than a decade ago.
The trial will be held in Guantanamo Bay, Cuba, in a military tribunal later this year.
The accused's lawyers say the court has no authority to judge them and they have been thwarted in their effort to give an adequate defence by military authorities.
If Osama bin Laden was the visionary behind al Qaeda, Khalid Sheikh Mohammed was the man who made his vision become reality, say prosecutors.
He is accused of coming up with the 9/11 attacks and meticulously organising their execution, recruiting the attackers and the back-up that allowed them to carry out the outrages.
But the case against him and the others is complicated by the years it has taken to come to trial and the methods used to bring them to justice.
The CIA has admitted waterboarding Sheikh Mohammed six times a day for a month, for instance.
He was deprived of sleep for more than a week.
None of the evidence acquired through those techniques will be admissible but critics say it makes the entire trial irredeemably tainted.
There has also been huge controversy over the location of the trial and its military nature.
The Obama administration had wanted a civilian trial on the US mainland but backed down under political pressure in the US Congress and because of public opposition to the plans.
It has reformed the military tribunal's procedures and says the court offers as good a chance of a fair trial as a civilian one.
It must now convince the world that the evidence presented against five men accused of such heinous crimes is admissible and the outcome of this trial is just.
They have the opportunity to plead their guilt or their innocence in crimes punishable by a death sentence.
In the past, the accused have said they want to die as martyrs.
A guilty plea would make a trial unnecessary, but a jury would still have to decide whether or not they deserve to be executed.