Holmes' legal team has already admitted that he was the shooter, but he has pleaded not guilty by reason of insanity. The jury will have to confront the question of whether Holmes was sane when he opened fire inside the movie theater. The mass shooting was one of the worst in U.S. history.
"I don't know if there's any closure for us because it is an open wound that I live with every day," Tom Sullivan, whose son Alex was among 12 people killed in 2012, tells Colorado Public Radio.
Colorado Public Radio reports, "Finding a juror who isn't affected by the events is expected to be a challenge."
The jury pool of 9,000 is one of the largest ever – three times the number for the Boston Marathon bombing trial.
The issues involved – the death penalty and an insanity plea – make this one of the most complex trials in Colorado history.
The Associated Press has a rundown of what will happen next:
"The first step begins on Tuesday, when 9,000 prospective jurors ... begin arriving at the courthouse in Centennial, in suburban Denver.
"Whittling those numbers to the 12 jurors and 12 alternates is expected to take months. The trial could last until October.
"In the 2-1/2 years since the shooting, the case has sparked an emotionally charged debate, with his parents begging for a plea deal that would save his life while many survivors and family members of victims have demanded that he be put to death."
If they find Homes guilty, jurors must decide whether to recommend the death penalty. If found not guilty, Holmes would be committed to the state mental hospital indefinitely.