This is a major moment in the history of Nigeria's young democracy: the first time an opposition candidate has ever beaten a sitting president, in this case President Goodluck Jonathan.
Nigeria is a young democracy. Since independence from the United Kingdom in 1960, the country has been mostly ruled by military juntas; civilian rule was only restored in 1999.
Since then, no sitting president has ever been kicked out of power by an opposition candidate. Nigerian politicians have often used the power of their office to sway things their way in elections.
If Jonathan leaves office peacefully, it would be a major victory for democracy in a country where elections are often plagued by rampant vote-buying, as well as the use of political thugs to intimidate or even kill rivals and to rig ballots.
Buhari's All Progressives Congress party ran by uniting several smaller opposition parties together and campaigning against Jonathan's lackluster record on the economy, corruption, and the fight against Boko Haram.
Buhari sought support from dissatisfied voters across Nigeria's diverse ethnic and religious groups, including people who might otherwise have voted for Jonathan's People's Democratic Party.
Nigerians are sick of the corruption that has spiraled under Jonathan — in one example of how bad things have gotten, the central bank chief admitted last year that $20 billion had disappeared from its reserves — and Buhari's strong anti-corruption message appears to have hit home.
Another factor that may have helped Buhari is that this vote appears to have been cleaner than past Nigerian elections. International observers have praised the election, saying new biometric voter ID card readers helped limit fraud.
