- Video report by ITV News Security Editor Rohit Kachroo
With Brexit talks looming the new prime minister will very quickly have to prove she is a tough negotiator.
Theresa May's reputation was forged by taking on the police over reform and standing up to other countries, including America, on extradition.
So, what do some of the people she fought for and against have to say about the PM-in-waiting?
Theresa May made her enemies during six years as Home Secretary - most notably with the Police Federation. In May 2014, she was booed and heckled at their annual gathering as she announced she would cut funding to the federation.
- Alexander Litvinenko
Earlier this year, after an inquiry into the death of Russian spy Alexander Litvinenko, May agreed to meet his widow who has been searching for the truth for a decade.
Marina Litvinenko says the meeting was not formal and that May was "absolutely real and a kind person."
- Gary McKinnon
In 2012, Theresa May refused to extradite Pentagon computer hacker Gary McKinnon to the US because of his Asperger syndrome.
She told the Commons that the move would contravene McKinnon's human rights.