When the confrontation was over, Martin lay dead and the debate over so-called "stand-your-ground" laws was on.
Zimmerman, 28, was driving through the neighborhood on a personal errand, observed Martin, 17, making his way home from a convenience store.
Zimmerman called police about 7 p.m. on a non-emergency line to report what he called a suspicious person in the Twin Lakes community.
While on the phone with a police dispatcher, Zimmerman apparently got out of his vehicle and followed Martin on foot, although the dispatcher had told him to remain in his vehicle.
Some time after Zimmerman got off the phone with the police, he caught up with Martin and a struggle ensued, resulting in Zimmerman fatally shooting Martin.
Zimmerman claimed self-defense and was not immediately arrested. Martin's family accused local police of backing Zimmerman's account of the slaying because Martin was African-American while Zimmerman is not.
By mid-March, the U.S. Justice Department got involved, promising "a thorough and independent review of all of the evidence and ... appropriate action at the conclusion of the investigation."
Sanford Police Chief Bill Lee told The Miami Herald his department "would welcome any outside entity that wants to come look at what we did."
"We have not done anything but conduct a fair and complete investigation," Lee said.
Lee temporarily stepped down from his position on March 22, after several days of protests across the country, calling for Zimmerman's arrest.
"My role as the leader of this agency has become a distraction from the investigation," Lee said in a statement. "It is apparent that my involvement in this matter is overshadowing the process.
"Therefore, I have come to the decision that I must temporarily relieve myself from the position as police chief for the city of Sanford."
Lee was fired June 20 after meeting with City Manager Norton Bonaparte.
President Barack Obama called the incident a "tragedy," saying it was "absolutely imperative that we investigate every aspect of this and that everybody pulls together -- federal, state and local -- to figure out exactly how this tragedy happened."
In mid-April, U.S. Attorney General Eric Holder said the Justice Department would look at potential civil rights violations in the case.
"If we find evidence of a potential federal criminal civil rights crime, we will take appropriate action, and at every step, the facts and law will guide us forward," Holder told the National Action Network.
On April 11, Zimmerman was charged with second-degree murder. He pleaded not guilty, saying he shot Martin in self defense.
At Zimmerman's bail hearing, he apologized to Martin's parents, saying: "I wanted to say I am sorry for the loss of your son. I did not know how old he was. I thought he was a little bit younger than I am. And I did not know if he was armed or not."
Circuit Judge Kenneth Lester set bond at $150,000 and ordered Zimmerman to wear an ankle monitor.
In May, Florida state Sen. Chris Smith, who formed a task force to review Florida's "stand-your-ground" law, proposed changing it to make it more difficult to claim self-defense.
"We wanted to make sure that we put together an accurate report, to give the governor direction, to give the Legislature direction and to give the governor's task force direction," Smith said. "Every day this law is being used and misused in courtrooms throughout the state of Florida."
Martin's parents, Tracy Martin and Sybrina Fulton, in October created a website and a group called "Change for Trayvon" to collect donations to "be distributed to candidates, elected officials and efforts which support the mission of... revising 'stand-your-ground' laws across the nation to ensure there is judicial or prosecutorial oversight," the site says.
On Dec. 19, Smith introduced a bill that would prevent armed citizens from automatically citing the "stand-your-ground" law.
"These commonsense changes are not about taking away gun rights from Floridians," Smith said in a statement. "It's about stemming the culture of violence in our society by putting responsibility for the consequences back into the hands of gun users, and anyone who deploys deadly force under 'stand-your ground.'"
"Stand-your-ground" laws, under which a person is justified in using deadly force in certain situations, have been challenged in other states in the wake of the Martin shooting, including Georgia, where a man sued the state to have the self-defense law nullified.
"It is not clear what actions would create 'reasonable belief' that deadly force is necessary," said the suit, filed by Michael Hutchins in U.S. District Court in Atlanta in April. "An individual seeking to stand their ground and assert self-defense has no way of knowing if their 'reasonable belief' comports with the standards protected by the law and [they] want to ensure that they do not subject themselves to criminal penalties."
In Michigan in May, Democratic lawmakers proposed repealing a 2006 law similar to Florida's.
"The 'stand-your-ground' law is the only law I can think of that actually promotes violence," Rep. Tim Bledsoe, D-Mich., said.
In August, Zimmerman's attorneys announced that they would seek a "stand-your-ground" hearing.
"The primary focus of a 'stand-you- ground' hearing is whether George Zimmerman reasonably believed that his use of a weapon was necessary to prevent great bodily harm to himself at the hands of Trayvon Martin," said defense attorney Mark O'Mara on Zimmerman's legal defense website.
Lawyers on the case in October said Zimmerman's trial, which is scheduled for June 10, will likely last about three weeks.