Iraqis are due to vote in the first parliamentary elections since the government declared victory over so-called Islamic State (IS) last year.
About 7,000 candidates from rival coalition alliances are vying for seats in the 329-member assembly.
Iraq is still struggling to rebuild itself after four years of war against IS, a BBC correspondent says.
He says whoever wins will need to keep Iraq's fragile unity in the face of sectarian and separatist tensions.
Polls across the country are due to open at 04:00 GMT and close at 15:00 GMT.
Iraqis can vote for rival lists of candidates. Most are predominantly Shia or Sunni, though the Kurds have their own lists.
The Shia-led government has won praise for the fight against IS militants, and security has vastly improved across the country.
But many Iraqis are disillusioned by widespread government corruption and a weak economy, the BBC's Martin Patience reports.
The vote also comes just days after US President Donald Trump pulled out of the Iran nuclear deal.
Some Iraqis fear their country could once again become a casualty in any struggle between America and Iran, our correspondent adds.