Buhari had a mixed record during his time in power
Former military ruler Muhammadu Buhari does not seem to know when to quit.
Although he lost badly to President Olusegun Obasanjo in the last elections which he rejected as rigged, Mr Buhari appears undeterred in his quest to be an elected head of state.
And having secured the nomination of the main opposition party, the All Nigeria Peoples Party (ANPP), he is once again edging closer to his goal.
Mr Buhari has already ruled Nigeria - from January 1984 until August 1985, in a period remembered for human rights abuses and a fight against corruption.
As a Muslim from Daura in Katsina State, he has had to deny allegations that he has a radical Islamist agenda given his support for Sharia law in the north.
This proved a problem for him in the 2003 polls, where he failed to secure much support among Christians in the south where he was viewed with some suspicion.
The verdict on Mr Buhari's 20 months as Nigeria's leader is mixed.
About 500 politicians, officials and businessmen were jailed as part of a campaign against waste and corruption.
Military ruler of Nigeria from 1984 to 1985
Deposed in a coup
Poor human rights record
Seen as incorruptible
Muslim from northern Nigeria
Some saw this as the heavy handed repression of military rule.
But others, and not just northern Muslims, remember it as a praiseworthy attempt to fight the endemic graft which was preventing Nigeria's development.
Largely because of this campaign, he retains a rare reputation for honesty among Nigeria's politicians, both military and civilian.
He also introduced a notorious decree to restrict press freedom, under which two journalists were jailed.
As part of his "War Against Indiscipline", he ordered the notoriously unruly Nigerians to form neat queues at bus stops, under the sharp eyes of whip-wielding soldiers.
Civil servants who were late for work were publicly humiliated by being forced to do "frog jumps".
However, his attempts to rebalance public finances by curbing imports led to the closure of businesses and many job losses.
Prices rose, while living standards fell, leading to a palace coup by Gen Ibrahim Babangida on 27 August 1985. Mr Buhari was imprisoned for 40 months.
Mr Babangida, who still retains an active role in Nigeria's politics, also wanted to speed up the restoration of civilian rule, which Mr Buhari did not see as a priority.
1937: Born in south-west; a Christian and a Yoruba
1976-79: Military ruler
1995-98: Jailed by military regime
1999: Elected president
Even now, Mr Buhari defends his military coup of 31 December 1983.
"It is up to the people. If you choose correct leadership, there won't be any need for the military regime.
"The military came in when it was absolutely necessary and the elected people had failed the country," he said last October.
When Mr Obasanjo was a military ruler in the 1970s, Mr Buhari held the key post of minister of petroleum affairs but there is now little love lost between the former colleagues.
His coup ousted Nigeria's last civilian government, led by Alhaji Shehu Shagari, who won elections organised by Mr Obasanjo.
And this has led to continuing questions about Mr Buhari's commitment to democracy.