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 clear front-runner with centrist Ahn Cheol-soo his nearest challenger.
The election is being closely watched at a time of economic uncertainty and heightened tensions with North Korea.
Mr Moon wants to increase contact with the North in contrast to ex-leader Park Geun-hye who cut almost all ties.
Observers are expecting a high turnout at the ballot boxes, with numbers boosted by younger voters, as South Koreans choose from 13 candidates.
Polls close at 2000 local time (1100 GMT), with the winner expected to be announced soon after.