This year's election is the closest since military rule ended in the West African nation 16 years ago, analysts say.
There are 14 presidential candidates on the ballot,
but the race comes down to a rematch between incumbent President Goodluck Jonathan and former military ruler Muhammadu Buhari. The two faced off in the last election in 2011.
In addition to the presidential elections, Nigerians will vote for governors in 36 states.
Though the stakes are high in this election amid intensified attacks by extremist group Boko Haram, there are other issues at play, including the economy and corruption.
Nigeria overtook South Africa as the region's largest economy last year.
But many complain the country's vast wealth from oil exports does not trickle down to the average citizen.
As many as 70% of Nigerians live below the poverty line and survive on less than a dollar a day.
Security takes center stage
Nigeria is under economic pressure because of falling crude oil prices worldwide and a weakened currency. Corruption has been a hindrance to building a stable economy despite years of democracy, analysts say.
But security has taken center stage as Boko Haram seeks to extend its tentacles with its recent pledge of allegiance to ISIS.
Just this year alone, the extremists have killed at least 1,000 civilians, Human Rights Watch says
One of the militants' most brutal acts was the kidnapping of nearly 300 schoolgirls
last April, a majority of whom are still missing. Boko Haram has become increasingly brazen, seizing towns in bloody attacks and declaring them Islamic caliphates.
'Successes here and there'
Nigeria has teamed up with neighboring Chad and Cameroon in a counteroffensive against the terror group. It appears to be working, with its military saying it has recaptured several key
Although the current government is "having successes here and there" in its fight against Boko Haram, it's not winning the war against the terror group, says Ayo Johnson, a documentary filmmaker and analyst on African affairs.
"This election will come down to who can protect Nigeria," Johnson says. "Who can make Nigerians feel safe."
Postponed for weeks
Jonathan rode a wave of popularity in 2011, when he portrayed himself as a man of the people. During campaigns, he talked about growing up without shoes, a message that resonated with average Nigerians. But in recent years, his popularity has plummeted, with Nigerians saying he has not delivered on his promises for change.
Buhari, a retired general, has unsuccessfully run for election three times. He ruled Nigeria in the 1980s after a military coup, and has appealed to those who have run out of patience with the current government.
His military background could be a plus or a minus.
"Many Nigerians will not forget he was a military leader, during a dictatorship," Johnson says. "Or maybe they will feel that they need a military leader to address fundamental problems such as terrorism."
The election was originally scheduled for February 14, but was delayed for six weeks amid attacks by the Islamist terror group.
To avoid a runoff, a candidate must get more than 50% of the vote and at least a quarter in two-thirds of the states.
If no candidate wins, a runoff election will be held seven days later.