The families will only be able to see each other for a few hours over three days.
A hundred people had been selected by each side, but some dropped out after realising the relatives they had hoped to see were no longer alive.
On Monday morning, numerous South Korean participants were in wheelchairs or helped by family members or support staff as they arrived at a Seoul-run front-line immigration office. Some experts say warming inter-Korean relations could suffer a setback if the North refuses to accept a US -led call for complete nuclear disarmament, and that is expected to figure into another inter-Korean summit set for next month in Pyongyang.
The families have been given the rare opportunity to meet again amid a diplomatic thaw between Seoul and Pyongyang that saw the leaders of the two countries meet in April for a historic first summit.
The older man stared at the picture silently, deep in thought, while his North Korean sister quietly wiped tears from her eyes.
"I remember how attractive you were", Cho Hye-do told her big sister.
"I don't know when he will die".
"I've lived this long to meet you", the 85-year-old said as she wiped away tears.
But as those who remember the war grow old, time is running out.
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The pain felt by the families split by the Korean War is one of the most visible legacies of the conflict which, 68 years after it began, still hasn't technically ended.
Since 1988, more than 132,000 people have registered with the Red Cross in South Korea for the reunion programme.
Most of the participants in the reunions are in their 70s or older and are eager to see their loved ones once more before they die.
Families at previous reunions have often found it a bitter-sweet experience, with some complaining about the short time they were allowed together and others lamenting the ideological gaps between them after decades apart.
Who is attending the meeting?
Park Hong-seo, 88, had hoped to see his older brother for the first time since 1946, when numerous family fled south to avoid communist rule.
Moon is himself a member of a divided family: His parents fled on a ship from the North Korean port of Hungnam in December 1950, and he accompanied his mother to meet her younger sister during an earlier family reunion in 2004.
"It is a shame for both governments in the South and the North that numerous families have passed away without knowing whether or not their lost relatives were alive", he said.
South Korea has also taken an active role in trying to broker talks between Pyongyang and Washington.
For Lee Jong-shik, 81, Monday's reunion was a hard-won second chance to track down his younger brother, Ri Chong Song, after the failure of a 2009 effort when a different individual showed up, to the dismay of the family from the South. Trump then met Kim in Singapore in June, although there has since been little indication that the North Koreans are genuinely willing to abandon their nuclear program.