Abdul Malik al-Houthi
Abdul Malik Al-Houthi is the head of the Houthis, a rebel group from the north of Yemen adhering to the Zaidi sect of Shia Islam.
Al-Houthi addressed the nation on a local television station on 20 January after his forces seized the presidential palace in Sanaa and attacked the private residence of ousted President Abd Rabbuh Mansur Hadi.
The Houthis declared themselves in full control of the government in February and replaced parliament with a Revolutionary Committee to act as Yemens interim authority.
The rebel group now controls large areas of main population centre in the west of the country and has been fighting over areas Yemens second city of Aden.
Abdrabbu Mansour al-Hadi
Abdrabbu Mansour al-Hadi was appointed president of Yemen in 27 February 2012 after his predecessor Ali Abdullah Saleh was ousted in the Yemeni uprising in 2011. He was the only candidate in the election.
Al-Hadi resigned on 21 January as the Houthis increased their control of the countrys political apparatus. In February, Al-Hadi fled house arrest in Sanaa to his hometown of Aden, which was then outside of the Houthis influence.
The ousted president arrived in Riyadh on 26 March as Saudi Arabia and its alliance of Sunni Arab governments started a bombing campaign against the Houthis.
In April, Al-Hadi wrote an editorial in the New York Times saying the Houthis are puppets of the Iranian government and must be stopped.
Ali Abdullah Saleh
Ali Abdullah Saleh was president of Yemen between 1990 and 2012, having previously served as president of North Yemen from 1978 until the unification that formed the present country.
Saleh resigned after the popular uprising in Yemen and formally ceded power to his deputy Abdrabbu Mansour al-Hadi on 27 February 2012.
The former presidents whereabouts are unknown. However, Saleh has actively used his influence among the military and tribes in Yemen to help the Houthis take control of the country.
Many suspect Saleh is using the Houthis in a bid to try to regain the presidency. Saleh offered to negotiate a political settlement between Saudi Arabia and the Houthis after the former began its bombing campaign in late March.
Al-Qaeda in the Arabian Peninsula
Al-Qaeda in the Arabian Peninsula (AQAP) is the Yemen-based branch of the Jihadist group founded by Osama bin Laden, described by the US government as its most dangerous spin-off.
The Sunni Islamist group is embedded in the south and southeast of Yemen and has launched many attacks during the civil war.
AQAP had repeatedly been a target for US drone strikes and has benefited from the departure of US Special Forces.
AQAP opposes both the Houthi rebels and the Hadis ousted government.
Islamic State in Iraq and Syria
The Islamic State in Iraq and Syria (Isis) has established a branch in Yemen with the aim of overthrowing AQAP as the dominant Sunni Islamist militant force in the country.
The organisation controls large areas of Syria and northern and western Iraq, and has been successful in recruiting foreign fighters from the Middle East and the West.
Isis claims to be behind suicide bomb attacks at two Shia mosques in March, killing at least 142 people.
Saudi Arabia-led Arab coalition
Saudi Arabia led a US-backed coalition of nine Sunni Arab governments to launch air strikes against Houthi positions in Yemen.
The offensive, codenamed Operation Decisive Storm, started on 25 March but was largely unsuccessful in dislodging the Houthis control. It was ended late on 21 April, with Saudi Arabia stating the campaign had acheived its objectives. The kingdom said it was moving on to Operation Restoring Hope, aimed at reviving diplomacy, repatriating foreign nationals and reviving aid.
The coalition included Saudi Arabia, Bahrain, Kuwait, Qatar, the UAE, Sudan, Morocco, Egypt and Jordan. Saudi Arabia asked Pakistan to support the offensive, but its parliament voted to remain neutral.
Iran has been accused by Al-Hadi, the US and Saudi Arabia of providing military and financial assistance to the Houthis. Tehran has denied giving any assistance.
The US has relocated warships to the coast of Yemen following reports of shipments of Iranian supplies heading for Yemeni ports.
Yemen and Iran have signed an agreement to establish 14 direct flights a week between the two countries, with the first landing in Sanaa at the beginning of March.