The Act was signed off by the Queen following eleven months of parliamentary scrutiny.
The Royal Town Planning Institute congratulated ministers on improving the Bill but said "the real test of the Localism Act would be its implementation and the resources made available to enable the planning system to deliver it".
The institute has called for more work on arrangements for safeguarding existing local plans and arrangements and on a planned period during which local authorities, the public and the development industry can "learn to work with the new regime and implement it in the most effective way".
And it said it would "continue the debate" on strategic planning and will support the development of effective practice even though the "duty to cooperate" has been strengthened.
RTPI president Richard Summers said: "Many issues still need to be clarified, some by legal challenge and others through guidance, but the key issue will be to reduce the continuing uncertainty, cost and delay for the planning system and the development industry."
The Department for Communities and Local Government said that the Localism Act would "trigger the biggest transfer of power in a generation".
Communities secretary Eric Pickles said: "Today marks the beginning of an historic shift of power from Whitehall to every community to take back control of their lives.
"The Localism Act pulls down the Whitehall barricades so it will no longer call the shots over communities - bug bears like housing targets and bin taxes are gone.
"For too long, local people were held back and ignored because Whitehall thought it knew best. That is changing for good."
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