Why is it important?
France is the EU's second-biggest economy and also one of the world's biggest military and diplomatic powers.
With two of the leading four top candidates hostile to the EU and NATO, the election could further shake up the West's liberal post-war order, already rattled by Britain's vote to leave the EU and Donald Trump's election to the White House.
How is the president elected?
The president is elected directly by the people in a vote of one or two rounds. If no candidate obtains an absolute majority in the first round, a run-off is held two weeks later. Every presidential election since 1965 has gone to a second round.
Who is running?
Eleven candidates spanning the spectrum from Trotskyist left to far-right are running.
The four favourites are far-right National Front (FN) leader Marine Le Pen, 48, centrist En Marche (On The Move) leader Emmanuel Macron, 39, conservative Republicans nominee Francois Fillon, 63, and hard left La France Insoumise (France Unbowed) leader Jean-Luc Melenchon, 65.
The other candidates are: Socialist nominee Benoit Hamon, 49; Philippe Poutou, a 50-year-old Ford factory worker; pro-sovereignty candidates Nicolas Dupont-Aignan, 56, Francois Asselineau, 59 and Jacques Cheminade, 75; Trotskyist economy teacher Nathalie Arthaud, 47 and Jean Lassalle, a 61-year-old MP and former shepherd.
How does the election work?
46.87 million voters are registered to vote. The country's 66,546 polling stations will open at 8:00 am (0600 GMT) and close at 7:00 pm (1700 GMT), an hour later than the last election in 2012.
In Paris and other big cities, the polls remain open until 8:00 pm (1800 GMT).
The election is the first in the history of France's 59-year-old Fifth Republic to take place under a state of emergency.
Over 50,000 police backed by 7,000 soldiers from the Sentinelle (Sentry) anti-terror operation will be on patrol during the vote.
When will we get the results?
Projections based on partial results usually come in at 8:00 pm but could be delayed due to the extra hour of voting.
The two top will go through to a runoff on May 7.
The next president will be sworn in by May 14 at the latest, taking over from Socialist President Francois Hollande.