Polls have opened across Mexico after a campaign marred by some of the worst political violence in the country for decades.
More than 130 candidates and political workers have been killed since campaigning began in September.
The presidential frontrunner is Andres Manuel Lopez Obrador, 64, the left-wing former mayor of Mexico City who has pledged to crack down on corruption.
He could oust the two parties that have governed Mexico for nearly a century.
This has been the most violent election campaign in Mexico in living memory. Now that voting day has arrived, however, many Mexicans see it as an opportunity to remove the government that has led the country to this point.
Millions of ordinary Mexicans are angry at President Enrique Peña Nieto and his administration, particularly over the sluggish economy and widespread corruption, crime and impunity.
The man widely expected to replace him, Andres Manuel Lopez Obrador who is often referred to just by his initials Amlo, was the runner-up in the last two presidential elections.
He has made tackling corruption the central plank of his election platform, promising to improve wages and pensions by stamping out rampant abuses by the state and the political and business elites.
His opponents, including the centre-right candidate, Ricardo Anaya, have tried to paint him as a populist and a dangerous maverick who cannot be trusted with the economy.
Most polls suggest, however, that a majority of voters have not listened to that message and are prepared to hand Lopez Obrador the presidency, third time around.