Hundreds of ex-Army personnel and the Royal Irish Regiment are taking part in a parade to remember the start of Operation Banner.
The parade is taking place in Lisburn and veterans are laying wreaths at three memorials.
The event marks the 50th anniversary of troops being deployed in Northern Ireland in August 1969 in what the Army refers to as Operation Banner.
Operation Banner lasted until 2007, costing the Army hundreds of lives.
It became the longest continuous campaign in British military history.
As part of the commemoration event, an Ulster Aviation Society Westland Scout Helicopter and old military vehicles are on display outside the Irish Linen Centre and Lisburn Museum.
Ian Simpson of the Northern Ireland Veterans Association which is hosting the event, said: “This 50th Anniversary of Operation Banner is a milestone in the history of Northern Ireland and we are honoured to hold this landmark event in Lisburn.”
He said it was a chance to remember those who died or were injured in the Troubles.
Mayor of Lisburn and Castlereagh City Council Alan Givan said: “Being a garrison city, it is fitting that Lisburn has been chosen by the Northern Ireland Veterans Association as the location to remember and commemorate those service men and women who tragically lost their lives and the many who were injured during this period. “
The Ministry of Defence said 722 soldiers were killed in the Troubles as a result of “hostile action”.
Deaths attributed to the Army number 297, according to Ulster University’s conflict archive. These include killings in controversial circumstances.
From the first deployment in August 1969, troop numbers swelled considerably over the years, peaking at 27,000.