BBC director general Mark Thompson has restructured the corporation's management and moved to strengthen its journalism in his first day in the job.
Mr Thompson's appointment comes at a crucial time for the BBC
A new board, led by his deputy Mark Byford, will oversee BBC journalism.
Mr Thompson replaces Greg Dyke, who resigned in the wake of the findings of the Hutton Report earlier this year.
He said: "We're going to have to change the BBC more rapidly and radically over the next three to five years than at any previous point in its history."
"It feels like the task of really changing the BBC has only begun."
Journalism was one of the BBC's "great glories", the former Channel 4 chief executive told staff on Tuesday.
But he said the corporation's journalism had "just been through the worst crisis in its 80-year history".
His comments were prompted by the Hutton inquiry, which concluded BBC reporter Andrew Gilligan's report on Radio 4's Today programme into weapons of mass destruction was "unfounded".
However a BBC internal inquiry said the proper process had been in place "but was not then followed" by Mr Gilligan, who resigned after Lord Hutton's report.
Mr Thompson said he did not want to dwell on the Hutton inquiry, and said: "My focus is on what we do now - what lessons do we learn, how do we go forward, putting it behind us but also minimising the chances of anything like it ever happening again."
He also stressed that ratings should never be the most important factor in BBC journalism or the corporation's other services.
Greg Dyke left the BBC in January
Mr Byford - who had been running the corporation since Mr Dyke's departure - is to lead staff in post-Hutton reforms and ensure news has high standards, while retaining journalistic independence.
He will chair the journalism board, which will be responsible for editorial policy in journalism and delivering "the most credible, the most impartial, the most engaging reporting in the world".
Mr Thompson will head another new board for creativity, which will decide the direction of programme-making and attempt to find ways of reaching new audiences.
"Without great programmes, great content, we're nothing," Mr Thompson said.
"Yet the key creative questions can seem like a bit of an afterthought at the top of the BBC."
Alan Yentob, current head of drama, entertainment and factual children's programmes, has been given the new title of creative director.
And a new commercial board will be run by current finance chief John Smith, who will also head a review of the BBC's commercial activities.
Mr Thompson said the BBC had to take a fresh look at value-for-money and costs.
"The period of buoyancy and rapid growth is over," he said. "The financial picture I see coming back in the BBC is a tight one.
"Every bit of the licence fee has been allocated to the end of current charter - and this year the BBC is spending more than its income."
He also replaced the 16-strong BBC executive committee - which runs the corporation - with an executive board with nine top managers.
Mr Thompson's appointment came during a crucial time for the BBC.
He takes up his post a week before the BBC's submission to the government over the corporation's charter review and the future of the licence fee.
Mr Thompson had originally ruled himself out of becoming director general, saying he would turn down any approach from the BBC.
At a press conference on his appointment on 21 May, however, he said the job was "a one-of-a-kind opportunity".
Mr Dyke and Michael Grade's predecessor as chairman, Gavyn Davies, left the corporation in the wake of Lord Hutton's report into events surrounding the death of weapons expert Dr David Kelly.