Twelfth of July parades are taking place in 18 locations across Northern Ireland.
Tens of thousands of people are expected at marches marking the anniversary of the Battle of the Boyne.
William III – the Dutch-born Protestant better known as William of Orange or King Billy – defeated the Catholic King James II in County Meath in July 1690.
On 12 July, marching bands from Orange lodges all over Northern Ireland parade through villages, towns and cities.
They then listen to speeches and prayers by senior Orangemen.
The day’s longest parade, in Belfast, stretches to six miles (9.5km) – nine districts will take part, accompanied by about 60 bands.
A wreath-laying ceremony took place at the Cenotaph at Belfast City Hall shortly before 10:30 BST.
Ahoghill will play host to one of the largest groups of Lambeg drummers on parade. The County Antrim village will resound to the drums’ distinctive rattle.
The County Armagh parade is the day’s largest gathering of Orangemen, with about 5,000 on parade in Tandragee.
The internationally-renowned mezzo-soprano Emma Brown was set to perform at the religious service.
The parade in the south Tyrone village of Augher sees Orangemen getting in the mood for the forthcoming Open golf championship being played at Portrush on the north coast.
The “Orange Golf Open” will see members competing in a “nearest the pin” competition.
The Orange Order was formed near Loughgall in County Armagh in 1795, when its founding members pledged their loyalty to the royal family and swore to defend the Protestant faith.
Senior Orangeman Edward Stevenson said the parades were a custom that had been handed down for over 200 years.
“Surely there is no other event on these islands that can bring such vast numbers of people onto the streets to enjoy our processions, either by taking part or simply to watch them go by,” the grand master of the Grand Orange Lodge of Ireland said.
While the Twelfth is the traditional pinnacle of the marching season, thousands of people have already enjoyed one of the annual pre-Twelfth highlights.
The sun shone brightly at the parade at the seaside village of Rossnowlagh in County Donegal on Saturday.
Orange Order members from both Northern Ireland and the Republic of Ireland took part in the march, which is traditionally held a few days before the main Twelfth of July events.
About 50 lodges from counties Cavan, Leitrim and Monaghan, as well as the host county of Donegal were on parade.
The full list of this year’s parades is as follows:
You can watch BBC Northern Ireland coverage of the parades in Belfast and Coleraine from on the BBC iPlayer, and there will be a highlights programme on BBC One Northern Ireland at 22:35.