STORY: The vote will decide not only who leads Turkey, a NATO-member country of 85 million, but also how it is governed, where its economy is headed amid a deep cost of living crisis, and the shape of its foreign policy, which has taken unpredictable turns.
Opinion polls give Erdogan's main challenger, Kemal Kilicdaroglu, who heads an alliance of six opposition parties, a slight lead, but if either of them fail to get more than 50% of the vote, there will be a runoff election on May 28.
The election takes place three months after earthquakes in southeast Turkey killed more than 50,000 people.
In the quake-hit town of Hatay, residents had to vote in makeshift polling stations.
Many in the affected provinces have expressed anger over the slow initial government response but there is little evidence that the issue has changed how people will vote.