The shooting led to weeks of protests that at times turned violent, and authorities fear a resumption of the angry protest when the jury's verdict is released.
Here is what you need to know about how the grand jury works and what they are considering:
What is the grand jury being asked to decide about?
The U.S. Attorney General's office is also investigating the incident and they could determine if the officer violated Brown's civil rights.
What evidence are they considering?
Most grand juries hear from just a few witnesses, including individuals with authority in the specific case, but not everyone involved. That is not the case in this grand jury. State prosecutor Robert McCulloch has said that he was planning on presenting "absolutely everything" to the jury.
"If it was a homicide case, the lead investigator may come in and tell what evidence exists. They may talk about the medical examiner’s report, maybe the grand jury will have a copy of it, but this [Michael Brown grand jury] is an unusual event because he [McCulloch] has called every witness and present every piece of evidence that exists,” civil rights attorney Steve Ryals told ABC News.
"I think in part because he wants to be able to say 'Look, you've accused me of being biased.' There is a concern that the grand jury will do whatever the prosecutor will tell the grand jury to do... He now can say ‘Look, I didn’t hold anything back. I presented everything. They [the jurors] had an option to ask for more if they wanted," Ryals said.
Do we know any of the witnesses that were asked to testify?
Officially, the list of witnesses is kept sealed, but through leaks, some of the witnesses have been reported. One whose testimony was widely publicized was the Brown family’s forensics expert, former New York City Chief Medical Examiner Dr. Michael Baden.
Another controversial witness reportedly was the officer, Darren Wilson. Ryals said the fact that the man at the center of the case was called in was "in and of itself unusual."
Ryals said that if the prosecutor kept his promise to include everything, Ryals suspects that the jury received testimony from “all of the witnesses at the scene, they would have heard from the police investigators. Aside from the witnesses, there would be a whole body of information around the forensic investigation -- where the shell casings landed, the blood spatter, that kind of thing,” Ryals said.
Is there one central conflict between the two sides of the case?
The most crucial disagreement was claims by bystanders that Brown had his hands up when the final shots were fired, while the officer and the police department claims the cop feared for his life as Brown kept advancing.
The police union and the Ferguson Police Department said that Wilson initially stopped Brown and a friend and told them to stop walking in the street and blocking traffic. It is not clear whether Wilson at some point became aware that Brown matched the description of a suspect in the theft of cigars from a nearby convenience store.
Police said that Wilson was assaulted by Brown while the officer was still sitting in his car. They claimed there was a struggle over the officer’s gun and that Wilson fired at least one shot inside the patrol car.
Brown ran off and the officer exited the patrol car. Dorian Johnson, who was with Brown at the time of the shooting, said the teenager did not attempt to grab the cops gun and that Brown ran from the officer.
What did the autopsies tell about the shooting?
Baden’s autopsy concluded that Brown was shot at least six times and that the final bullet struck Brown in the top of the head.
The county autopsy, according to the St. Louis Post Dispatch which obtained a copy of the document, suggested that Brown was shot in the hand at such a close range that there may have been gun powder burns, which could support Wilson’s description of a struggle over his gun.
Experts have said that the autopsy and forensic report are unlikely to be able to determine whether Brown’s hands were raised when he was shot.
Did Darren Wilson suffer any injuries during the incident?
Police have said Wilson was taken to the hospital with a serious facial injury. Hospital and police records show that Wilson was taken to the hospital. A video of Wilson taken the night of the shooting show no obvious facial injuries.
Who is on the grand jury?
The 12-person jury is made up of a group of St. Louis residents who would have been selected from the normal subset of people who turned up for jury duty. The make up of the jury is officially kept secret.
The decision to charge Wilson with a crime does not need to be unanimous, Ryals said, noting that only nine jurors are needed to come to a conclusion.
What has their life been like since joining the grand jury?
Unlike certain criminal trials, the grand jury has not been sequestered, meaning that they have inevitably been exposed to the coverage and debate over the case. Part of the reason for the lack of sequestration is the fact that the grand jury reportedly only meets about once a week, usually on Wednesdays.