The 2011 presidential election: a better-run vote, followed by deadly violence
Goodluck Jonathan won with 58.9% in what observers called a significant improvement on 2007, but riots across 12 northern states killed more than 800 people.
The 2007 presidential election: Yar'Adua's victory and the election its winner said was flawed
Umaru Musa Yar'Adua won the first handover from one elected president to another, but observers called the polls the worst they had seen, and the new president agreed they needed fixing.
The 2003 presidential election: Obasanjo's re-election and the first civilian-run polls
The first election organised by a civilian government ended in a second term for Olusegun Obasanjo, amid serious complaints of fraud and violence.