Wales's senior civil servant, who has been permanent secretary of the Welsh Assembly Government since its creation, has announced his retirement.
Sir Jon says it "makes sense" for him to stand aside now
Sir Jon Shortridge, 60, will step down in April after nine years in office.
He said he wanted to allow time for his successor to gain experience in the job before First Minister Rhodri Morgan's expected departure in 2009.
Mr Morgan paid tribute to Sir Jon, saying he was "a major partner in the success of devolution in Wales".
"As permanent secretary to the Welsh Office and then the Welsh Assembly Government, he has led the civil service of both organisations through dramatic and unprecedented change," said Mr Morgan.
"His massive contribution is immeasurable in ensuring that the organisation has developed and adapted to the new demands of devolved government".
Sir Jon had steered the public service in Wales through major changes, said Mr Morgan, including the Government of Wales Act 2006 which conferred new legislative responsibilities on the assembly government.
"Jon has played a key role in supporting the establishment of what is a radically different system of government in Wales.
"He leaves behind him a strong administrative machine firmly embedded within Welsh civic life," said Mr Morgan.
Sir Jon, who was knighted in 2002, became permanent secretary of the Welsh Office in March 1999, and continued those duties when the Welsh assembly was established two months later.
He first joined the Welsh Office in 1984, where his duties included responsibility for reorganising local government in Wales, setting up the north Wales child abuse inquiry, and establishing the assembly.
Sir Jon said that by next April he would be the longest serving of the present crop of permanent secretaries in the UK, and wanted to allow time for his successor to gain experience in the job before Mr Morgan's successor is appointed.
"I am very proud of what has been achieved over the past nine years and believe that we have secured a firm foundation for devolution in Wales," said Sir Jon.
"By April, the new coalition assembly government should be well established and it will make sense for me to stand aside and allow a new permanent secretary into the next phase of its development."
His successor will be appointed following an open competition to be held later this year.