South Koreans are voting in a presidential election, called early after a huge corruption scandal brought down their former leader.
Liberal Moon Jae-in is the strong favourite with centrist Ahn Cheol-soo his nearest challenger.
Economic issues are a big concern for voters but the election could see a shift in policy towards North Korea.
Mr Moon wants to increase contact with the North in contrast to impeached Park Geun-hye who cut almost all ties.
A record turnout is predicted, with numbers boosted by younger voters, as South Koreans choose from 13 candidates.
Polls close at 20:00 local time (11:00 GMT), with the winner expected to be announced soon after. The new leader is likely to be sworn-in on Wednesday.
How might the vote affect North Korean ties?
Heightened tensions on the Korean peninsula in recent weeks have made the perennial worries over the South’s volatile neighbour a key issue.
Mr Moon, of the Democratic Party of Korea, has advocated greater dialogue with North Korea while maintaining pressure and sanctions.
Both he and Mr Ahn have urged US President Donald Trump to cool his rhetoric towards the North after his administration suggested it could take military action over Pyongyang’s weapons programme.
Hong Joon-pyo of the conservative governing Liberty Korea Party however has attacked Mr Moon’s approach, saying on Thursday morning that the election was a “war of regime choices”.
North Korea state media said it favoured a return to an earlier era of communication and co-operation known as the Sunshine policy, seen as an endorsement of Mr Moon who was part of the previous South Korean government which promoted that policy.
What are the other concerns?
All the candidates are promising to protect the fragile recovery in South Korea’s economy – the fourth largest in Asia – and to bring down youth unemployment, which remains stubbornly high.
There have been vows to reform the family-run conglomerates – chaebols – which dominate the domestic economy.
Whoever wins will have to tackle ties with China, which retaliated economically over the deployment of a US missile defence system in the South.
Candidates have been promising a break from the past as symbolised by the deeply unpopular Ms Park.
A 22-year-old voter said: “I voted with the hope of not electing the same kind of president again in future.”