The trial for James Forcillo, the Toronto police officer charged in the 2013 fatal shooting of 18-year-old Sammy Yatim on a TTC streetcar begins today in the Superior Court of Justice in Toronto. 

Forcillo pleaded not guilty to charges of second-degree murder and attempted murder in September.

Here's a primer of what you need to know as the trial begins:

About Sammy Yatim

Yatim, 18, was shot to death on July 27, 2013, after police responded to reports of a passenger carrying a weapon on a streetcar stopped on Dundas Street West near Bellwoods Avenue in downtown Toronto. 

Jurors will have to consider evidence from the Crown and the defence around what happened between the time officers arrived at the stopped streetcar and when Yatim was shot.

The teen was taken to St. Michael's Hospital, where he was pronounced dead that night.

About James Forcillo

YEAR Diary Photos

Toronto police Const. James Forcillo is facing second-degree murder and attempted murder charges in the death of Sammy Yatim. (Nathan Denette/Canadian Press)

 Less than four weeks after the shooting, following an investigation by Ontario's Special Investigations Unit, Forcillo was charged.

The SIU, which investigates when police officers are involved in incidents where someone has been seriously injured, dies or alleges sexual assault, acted with what had been noted as uncharacteristic speed in the shooting. 

Lawyer Peter Brauti, who is representing Forcillo, regularly represents police officers in court.

What's at stake?

It's rare to see a Toronto police officer charged with second-degree murder. This is only the second time it has happened. Some legal experts have said it's also unusual that someone would be charged with second-degree murder and attempt to commit murder. 

Second-degree murder involves a deliberate intent to kill but does not involve planning. It carries a sentence of life in prison with no parole for 10 years. However, parole ineligibility could extend for 25 years. Those decisions are at the judge's discretion.  In this case, Justice Edward Then is presiding. 

The second charge is based on allegations that a firearm was used in an attempt to commit murder. If convicted of attempted murder, Forcillo would face a minimum sentence of four years since a gun was used.

What's happened since the shooting?

Besides the criminal proceedings, Yatim's family is suing Forcillo, the Toronto Police Service and the Toronto Police Services Board in a separate lawsuit for $7 million. The lawsuit alleged police acted with reckless disregard for Yatim's life.