The changes have proved extremely controversial
Government reforms of the NHS in England have come into force and health leaders warn of a tough year ahead.
Monday marks the first day of the new structures.
GP-led groups have taken control of local budgets and a new board, NHS England, has started overseeing the day-to-day running of services.
The NHS Confederation said the reforms represented a big opportunity but should not be seen as a "silver bullet" for the challenges ahead.
Mike Farrar, chief executive of the confederation, which represents health managers, said the squeeze on finances and the need to rebuild public confidence after the Stafford Hospital scandal meant the NHS was facing a critical period.
He said the reforms would bring clinical expertise to the fore of decision making, which would be a "huge asset".
But he warned: "We need to recognise the huge challenges facing the health service. New structures alone won't enable us to tackle these challenges, and we should not see them as a silver bullet.
"Those doing the day-job face major pressures in trying to keep the NHS's head above water, while focusing on making the new world work."
The start of the new system comes nearly three years since the changes were put forward.
The publication of the plans in the summer of 2010 sparked a long and, at times, damaging battle for the government to push through with its changes.
Ministers even had to take the unprecedented step of halting the progress of the bill through Parliament amid criticism from medical bodies, academics and unions.
In particular, concerns have been expressed about what many believe is a greater role for the private sector.
'Compassionate care'
Some have also questioned whether introducing such major changes - they have been dubbed the most radical overhaul since the NHS was created - at a time when money is so tight makes sense.
But as the new bodies take up control - and the old organisations, including 152 primary care trusts, are scrapped - the government maintained the changes would put the NHS on a firm footing for the 21st century.
Health Minister Anna Soubry said: "The health service will improve, work smarter and, importantly, build an NHS that delivers high quality, compassionate care for patients."
But shadow health secretary Andy Burnham predicted the changes would have the opposite effect.
"Far from letting 'doctors decide', ministers are forcing the medical profession to open up all NHS services to the market.
"Hundreds of new private companies now risk fragmenting patient care when more integration is needed."
GP Catherine Briggs said she would welcome more control over how budgets are spent.
"Because GPs have face-to-face contact with patients every day and because they know their patients and their communities really well," she said.
"That means they are really well-placed to be able to make decisions about how healthcare should be delivered best."
But GP John Hughes said he had reservations.
"The GPs aren't really free to do what they like with the money as a lot of people seem to think," he said.
"Most of the directions as to what happens to that money and what should be bought or commissioned locally is coming from the Department of Health.
