United States presidential hopeful Barack Obama has introduced veteran Democratic Senator Joe Biden as his running mate at a rally in Springfield, Illinois.
Mr Obama hailed Mr Biden as a "man with a distinguished record and a fundamental decency".
He confirmed his choice of running mate on his website after the news began to leak to the media.
At the place where he launched his presidential campaign 18 months ago, Mr Obama outlined Mr Biden's accomplishments in the Senate, his blue collar roots and his experience on foreign policy.
"He's an expert on foreign policy whose heart and values are rooted firmly in the middle class," Mr Obama said.
He also emphasised Mr Biden's drive for change, despite his 30 years spent in the Capital.
"For decades, he has brought change to Washington, but Washington hasn't changed him," Mr Obama said.
Mr Biden referred to his own short-lived bid for the White House against Mr Obama for the 2008 nomination, before dropping out in January.
"You learn about a man when you debate with him, you see how he thinks. Barack Obama has the vision and courage to make this a better place. He is a clear-eyed pragmatist who will get the job done," he said.
Mr Biden, a 65-year-old veteran lawmaker, is highly respected on foreign policy and is a six-term senator who serves on the powerful Foreign Affairs Select Committee.
He has represented the state of Delaware in the US Senate since 1972.
The Democratic campaign will be hoping his presence will reassure voters who are concerned about Barack Obama's relative inexperience, particularly in the international arena.
And, crucially, Senator Biden also appeals to working-class Americans and hails from Pennsylvania, a key swing state in this election, our correspondent adds.
Republican presidential hopeful John McCain's camp called the choice of Mr Biden an admission by Barack Obama that he was not ready to be president.
Mr McCain has reportedly not yet settled on a running mate.