President Asif Ali Zardari
One of Pakistan's most controversial political figures, Asif Ali Zardari is often considered something of an accidental president. He came to power in September 2008 on a wave of public sympathy following the assassination of his wife, former Prime Minister Benazir Bhutto. But his term in office has been one of unrelenting political and social turmoil and growing instability.
His leadership has seen a marked deterioration in Islamabad's relationship with Washington, with the US questioning whether Islamabad is doing enough to tackle militancy.
His career has also been tainted with corruption allegations. After removing an amnesty in 2009, the Supreme Court is once again pushing to reopen a corruption case for which he has already spent eight years in prison. He is increasingly unpopular with the Pakistani public, and has been plagued with health problems which have taken him abroad for treatment at crucial moments.
The latest test to his leadership has been the leaking of the controversial memo asking for US help to avert a coup, following the Bin Laden raid. Mr Zardari denies involvement, but it is being investigated by the Supreme Court and if found responsible, he faces possible impeachment.
Prime Minister Yousuf Raza Gilani
Often considered the quiet man in Pakistani politics, Yousuf Raza Gilani has been increasingly vocal in his criticism of the military in recent weeks. In late December, he warned of conspiracies to overthrow the government - seen as a reference to Pakistan's military. "There can't be a state within the state. They have to be answerable to this parliament," he said. The army rejected his accusations.
But days later, Mr Gilani criticised the the army and intelligence chiefs for providing evidence to the memo inquiry implicating the government. When the army said his comments could have "grievous consequences for the country", he responded by sacking his defence secretary.
The Supreme Court, meanwhile, has said Mr Gilani could be disqualified from office for not reopening corruption cases against top politicians, including Mr Zardari.
Gen Ashfaq Kayani - Army chief
As military chief, Gen Kayani has presided over one of the most turbulent periods in Pakistan's military history, with the killing of Osama Bin Laden, the growing threat of terror attacks and widespread public anger at drone strikes which have resulted in the deaths of civilians as well as militants.
He has pushed for an investigation into the memo affair, saying the alleged note was a conspiracy against the army. In return, he was accused by Mr Gilani of acting unconstitutionally.
Pakistan's military has carried out three coups in the past and there has been mounting concern that another is on the cards. But analysts say the military has little to gain from such action - it has its hands full with the fight against Islamist militants and such a coup would attract strong international criticism.
Gen Pervez Musharraf - former leader
Pakistan's last military leader has been living in self-imposed exile since stepping down in 2008. But he remains a potent figure in Pakistani politics and in early January, announced he would return to the country within weeks, telling his supporters to prepare for elections. He faces immediate arrest on his return however, over charges he failed to provide adequate security for former Prime Minister Benazir Bhutto, which contributed to her assassination in 2008.
The military is likely to view his return with suspicion, however, and correspondents say the image of military rule was so bad by the time he left office, the public are not expected to rush to his support.
Nawaz Sharif - opposition leader
Twice prime minister in the past, Nawaz Sharif remains the main political force in the Punjab, the most populous region of Pakistan. He is the president of Pakistan's second largest party, the Pakistan Muslim League-N. Many thought he would have won the 2008 elections, had Mr Zardari not been given a polls boost by the death of his wife.
Mr Sharif has been accused in the past of being too friendly an opposition to the PPP and failing to seize opportunities to challenge them. But correspondents say he has perhaps played a careful game, waiting until the time was right to be more forceful.
It was Mr Nawaz who brought the alleged memo to the US to the attention of the Supreme Court - he described it as a "treasonous" act and he accused President Zardari of being behind it, along with former Pakistani ambassador to the US Husain Haqqani, who has since resigned.
Imran Khan - leader of Movement for Justice
The former international cricketer has been on the political scene for several years, leading his Pakistan Tehreek-e-Insaf (Movement for Justice) party, but has struggled to make any significant gains. But in recent weeks he has been experiencing a surge in popularity, riding a wave of disillusionment, particularly among the urban middle class.
He has also won the support of politicians who have grown disgruntled with their own parties - they could help counter accusations of political inexperience.
Mr Khan has pledged to sweep away the rampant corruption plaguing Pakistani politics in part by calling for an end to foreign aid. But correspondents say he has some way to go to turn popular support into electoral gains. He is, however, believed to be popular with the military.
Mohammad Iftikhar Chaudhry - Chief justice
Pakistan's chief justice has form in opposing the sitting government. He was one of several judges sacked by Gen Pervez Musharraf in 2007 after they questioned his right to remain in office. A long campaign was launched and he was finally restored to his post in March 2009.
For a time he had a strong populist image, seen as a champion of the rule of law, praised as the only judge in history to have stood up to a military ruler and won. But he has been accused of being selective in the cases he pursues.
The Supreme Court has historically given legitimacy to military coups and some say he has dragged his feet over corruption allegations against the intelligence service while pursuing those against the government. The court is now pursuing the government over the amnesty against corruption prosecutions as well as over the memo. The view now is that the military could be sitting back to let the court do the work of ousting the government.
Lt Gen Ahmad Shuja Pasha - Intelligence chief
The head of Pakistan's shadowy secret service, the ISI, faced widespread calls for his resignation following the killing of Osama Bin Laden by US special forces in May 2011. Many said it was inconceivable that his organisation had not known the al-Qaeda head was living undetected in Pakistan. The ISI has for some time been accused of turning a blind eye or even supporting militant groups.
Just weeks after the alleged memo emerged, Lt Gen Pasha travelled to London to meet the man who allegedly drafted it, Pakistani-American businessman Mansoor Ijaz. He submitted a statement to the Supreme Court saying he believed there was enough evidence of a conspiracy against the army. In turn he has been accused of acting unconstitutionally by apparently travelling to London with the authorisation of the military, but not the government.
President Zardari is known to strongly dislike his intelligence chief, but appears unwilling to become embroiled in direct confrontation. Lt Gen Pasha has only a few months left on his tenure, and both parties seem keen for him to see it out and leave quietly when it ends.
This is how the Brits view our leaders - thought it maybe of interest
I think the military backing of Imran Khan is paramount for the future and the pair score wellin the future
BBC News - Pakistan political crisis: Key players