Carly Fiorina and Ben Carson launch Republican 2016 bids
Carly Fiorina, the former chief executive of Hewlett-Packard, and neurosurgeon Ben Carson have joined the growing 2016 Republican field.
Ms Fiorina is the first female in her party to enter the White House race and Mr Carson the first African American.
Both are political outsiders, unlike the senators and governors they now come up against for the nomination.
Democrat Hillary Clinton and Republican Jeb Bush, who has not yet officially entered, are among the frontrunners.
Ms Clinton was immediately targeted by Ms Fiorina when she made her announcement on ABC's Good Morning America programme.
"I have a lot of admiration for Hillary Clinton, but she clearly is not trustworthy," Ms Fiorina said, pointing to the recent controversy over Ms Clinton's use of a private email server when secretary of state.
Fiorina analysis - Anthony Zurcher, BBC News
Carly Fiorina lost her only run for public office, a 2010 Senate race in California, by 10%. Her primary claim to fame is serving as head of computing giant Hewlett-Packard, a job from which she was ousted after six years amid stockholder unrest.
Why is she running? A cynic would point to her book, which is set to go on sale the day after her announcement, and argue she is more interested in publicity than getting votes.
Advocates will point to the fact that she is almost certainly the only woman to seek the Republican nomination this year, with Fortune 50 business experience, and is well positioned to launch attacks on Democratic front-runner Hillary Clinton without confronting charges of sexism.
Ms Fiorina has been well received during her appearances before Republican audiences in the run-up to her announcement. Now we'll see if she can translate those good feelings into more tangible signs of support.
Retired surgeon Ben Carson chose Detroit Music Hall, with a choral version of Eminem's "Lose Yourself", to make his announcement.
He outlined his vision of the US as a "place of dreams'' where people can thrive when they escape the restrictions of government.
"It's time for people to rise up and take the government back," he said. "The political class won't like me saying things like that. The political class comes from both parties."
Carson analysis - Anthony Zurcher, BBC News
Mr Carson has stumbled at times in media interviews, a sign of campaign inexperience.
But controversial answers on hot-button issues like evolution, healthcare reform and gay rights have done little to damage his positive perception among core conservative voters.
If enough of them stand by Mr Carson come next year, he could at the very least prove to be a spoiler for other socially conservative candidates, like Texas Senator Ted Cruz and former Arkansas Governor Mike Huckabee.
Three other Republicans have already entered the race - senators Ted Cruz, Rand Paul and Marco Rubio.
On the Democratic side, Ms Clinton will need to overcome the challenge of left-wing Vermont Senator Bernie Sanders.