Right-winger Ivan Duque leads after the first round, well ahead of leftist Gustavo Petro.
At stake in the second-round vote is the future of the peace deal signed by the government and the Revolutionary Armed Forces of Colombia (FARC) rebels in 2016.
Duque is opposed to it, saying it is too lenient on FARC. Petro backed it, but needs double the vote he got in the first round.
And if Duque wins the runoff vote, will it reignite an armed conflict that saw more than 200,000 people killed over five decades?
Presenter: Hashem Ahelbarra
Gimena Sanchez – Colombia human rights advocate at the Washington Office on Latin America
Peter Hakim – president emeritus and senior fellow of the Inter-American Dialogue
Source: Al Jazeera News