WASHINGTON -- President Barack Obama’s first task as he launches his second term will be to fill a number of key vacancies expected in the White House, positions that will be instrumental in helping him tackle his legislative agenda as well as navigate conflicts around the globe.

Already, a pair of high-profile officials – Secretary of State Hillary Clinton and Treasury Secretary Timothy Geithner – have said they’ll step down.

Obama also will have to find a new director of the CIA after accepting the resignation Friday of David Petraeus, who acknowledged an extramarital affair. The president named the agency's deputy director, Michael Morell, to serve as acting chief.

No other Cabinet members have said they plan to leave, but the heads of the departments of Defense, Justice and Transportation may quit in the coming months. The position of commerce secretary already is vacant.

Other Cabinet officials are expected to stay, including those who run the departments of Education, Housing and Urban Development, Veterans Affairs, and Health and Human Services.

While Obama’s Cabinet remained relatively stable in his first term, several senior aides left.

Political observers expect half of the president’s Cabinet – as well as a handful of senior staffers – to change at the start of his second term, as has been the nature of life and work at the White House. Former Presidents George W. Bush and Bill Clinton each replaced half of their Cabinets after they were re-elected.

Obama is also likely to look at rewarding some campaign aides with high-level positions. Here’s a look at some of the possible vacancies and potential replacements:

Treasury

Geithner, the longest-serving member of the president’s economic team, already has said he’ll leave.

This vacancy is arguably the most difficult to fill, as Congress and Obama face a deadline to ward off looming tax increases and spending cuts that threaten to throw the nation into another recession. The next secretary needs to be a name with clout on Capitol Hill, as well as one that global financial markets respect. Earlier this year, Geithner said on ABC that his replacement needed to be someone “willing to tell (Obama) the truth.”

Possible replacements include: Jack Lew, the president’s chief of staff and a former head of the Office of Management and Budget; Larry Fink, a founder of financial giant BlackRock, the largest money management firm in the world; Roger Altman, a deputy treasury secretary in the Clinton administration; and Erskine Bowles, former chief of staff to President Clinton and a co-head of Obama’s special commission that drafted a report on fixing the budget deficit and debt problem.

State

Hillary Clinton plans to step down, though she recently said she’d stay on until Obama picked a replacement. Her successor will have to deal with situations across the globe, including the unresolved debt crisis in Europe, the rise of militant Islamist factions in transitional Arab states, a nuclear showdown with Iran and violence in Syria.

Possible replacements: Sen. John Kerry, a Massachusetts Democrat who’s the chairman of the Senate Foreign Relations Committee; National Security Adviser Tom Donilon; Samantha Power, special assistant to the president and senior director for multilateral affairs and human rights; William Burns, the deputy secretary of state; and R. Nicholas Burns, a former undersecretary of state for political affairs.