As the first woman and the youngest person to be elected Alaska's state governor, Sarah Palin is used to breaking new ground.
The 44-year-old was elected in 2006 and has been credited with bringing in reforms in her two years in office.
Presenting her as his running mate in November's presidential election, John McCain described her as "someone with strong principles, a fighting spirit and deep compassion".
As well as portraying her as a devoted wife and mother-of-five, Mr McCain also stressed her executive experience and ability to stand up against entrenched interests and corruption.
"She has the grit, integrity, good sense and fierce devotion to the common good that is exactly what we need in Washington today," he said.
Mrs Palin is seen as boosting Mr McCain's ticket because she is a woman - and so may appeal to disappointed Hillary Clinton supporters - as well as being young, energetic and from outside the Washington political scene.
She is also viewed as having a strong social and fiscal conservative record. She opposes abortion and holds a life membership of the National Rifle Association.
She is the first woman Republican to be put forward as a vice-presidential candidate. The only other US female running mate was Democrat Geraldine Ferraro, in 1984.
