Yemeni President Ali Abdullah Saleh faces a challenge from four other candidates in Wednesday's presidential election, the first in seven years.
The strongest challenger to the incumbent from north Yemen is Faisal Bin Shamlan, whose background lies in the formerly communist south.
Here BBC Monitoring profiles the two men.
ALI ABDULLAH SALEH
Mr Saleh, leader of the General People's Congress (GPC), has been president of Yemen since unification in 1990, having previously served for 12 years as president of the northern Yemen Arab Republic (YAR).
President Saleh is the only leader the unified Yemen has ever known
Born in 1942 in Bayt al-Ahmar, near the capital Sanaa, Mr Saleh attended a local Koranic school before joining the north Yemeni army at the age of 16.
He then began a meteoric rise through the ranks, participating in the 1962 revolution in which army officers set up a republic.
In 1974, he helped a 10-member Military Command Council seize power in the YAR and, shortly afterwards, was appointed to serve in the military government in Taizz.
Four years later, when President Ahmed Hussein al-Ghashmi was assassinated by an envoy from the communist south, Mr Saleh suddenly rose to political prominence.
Parliament soon elected him president of the YAR and commander-in-chief.
Surrounded by a close circle of aides, including his brothers and members of his powerful Hashid tribal confederation, Mr Saleh won two further terms in office in 1983 and 1988.
At the same time, he worked for unification with the Soviet-backed south, finally achieving it on 22 May 1990, when he was appointed chairman of the Presidential Council of the new Republic of Yemen.
After crushing an uprising led by former communists in the south in 1993, he was re-elected president for another five years by parliament.
Since 1994, Mr Saleh has had to contend with a serious challenge from Islamists and militant groups, alienating some by backing the US-led "war on terror".
He has allowed the US military to attack militants based in Yemen and has closed hundreds of religious schools.
In 2004, he put down an armed revolt led by a cleric in a mountainous area of the north.
Though the president has pledged to continue the fight against Islamists, his government is as yet unable to extend its writ over many of Yemen's rural areas and public support for militants has not disappeared.
FAISAL UTHMAN BIN SHAMLAN
The main opposition candidate is a former oil minister with a reputation for integrity in public office.
Mr Shamlan heads a coalition of the main opposition parties
Born in the eastern province of Hadhramut in 1934, Mr Shamlan went to secondary school in Sudan before travelling to the UK to study civil engineering.
After finishing his degree, he returned to Yemen and helped found a branch of the Egyptian Islamist organisation, the Muslim Brotherhood, pushing for independence for southern Yemen from Saudi Arabia and Britain.
When the south gained independence in 1967 as the People's Republic of Southern Yemen, he became minister of works and communications in the first government until a coup in 1969.
Posts in the electricity board and oil industry followed.
Despite a successful career in commerce, Bin Shamlan remained a politician at heart and in 1971 he was elected an MP, a position he would hold until unification in 1990.
After unification, he returned to government as director-general of marketing at the oil ministry before entering politics in 1993 as an independent MP.
Despite being an independent, he was nominated by an Islamist party, the Yemeni Alliance for Reform (Islah), for the post of oil minister in a coalition with President Saleh's GPC party.
He built up a reputation for integrity, reportedly even resigning twice in protest at government corruption, but was replaced in 1995 when Islah lost ground to the GPC in a reshuffle.
He again took a stand in 2001, when he resigned as an MP in protest at the House of Representatives' decision to extend its mandate from four to six years.
Selected in July as the presidential candidate of the Joint Meeting Parties, a union of the main opposition parties, the 72-year-old said he aimed to combat corruption and install real parliamentary democracy.
BBC Monitoring selects and translates news from radio, television, press, news agencies and the internet from 150 countries in more than 70 languages. It is based in Caversham, UK, and has several bureaux abroad.