His peers describe him as that rare thing among academics: a man whose ideas have had a major influence in the classroom. He was once even able, it is claimed, to make PGCE courses memorable for his students.
Dylan Wiliam, one of Britain's leading educationists and a persistent critic of the Government's testing and accountability regime, is returning to this country after three years in America.
The co-author of a bestselling book which acted as a catalyst for schools'
drive towards assessment for learning, becomes deputy director of London University's Institute of Education from Monday.
Ministers may not be happy, having no doubt breathed a sigh of relief after Professor Wiliam left Britain in 2003 for a post in New Jersey as senior research director with a major American testing company.
Over the previous years, the 50-year-old, distinctive with his earring and penchant for spicing apparently dry statistical analyses with historical anecdotes on the perils of over-emphasising what can be measured, had acquired an enviably high profile.
A former maths teacher, he has also found time to foster 16 children with his wife Siobhan Leahy, a former secondary headteacher. His interests include baseball, scuba diving and restoring old houses.
With Professor Paul Black, his erstwhile colleague at King's College, London, he wrote Inside the Black Box, which claimed that GCSE results would improve if teachers used test information not to grade pupils but to tell them what they needed to do to improve. This is also known as assessment for learning. The book sold 40,000 copies and helped to persuade ministers to get teachers to make more use of assessment information in planning areas for future teaching.
Last month, The TES revealed how the Qualifications and Curriculum Authority has been investigating an assessment for learning approach which could be an alternative to national tests.
Professor Wiliam continued his work on teacher assessment in America, where he has been senior research director at Educational Testing Services, based in Princeton, New Jersey.
The company has been piloting assessment for learning approaches in schools in 28 American school districts, from California to Cleveland. Although Professor Wiliam said results were still being analysed, he said it appeared there were statistically significant improvements in pupils'
learning. The scheme is now going to be offered to all American schools, and he will continue to advise the company on the project as it is extended.
Professor Wiliam is a passionate advocate for professional development, pointing to research showing that teachers' skills make more difference to pupils' development than class sizes or the quality of a school.
His other great focus has been pointing out the fallibilities of national tests and exams in judging pupil performance. He has published papers suggesting that 30 per cent of pupils may be given the wrong test level, a finding which ministers have never disproved.
Stephen Gorard, of York university, said senior officials at the QCA had come round to the view that the Government needed to be more open about the tests' potential inaccuracy, partly because of Professor Wiliam's work.
Those who know him also respect the impact of his research.
Tim Oates, head of research at Cambridge Assessment, said: "He's fantastic.
He has done a series of high-quality research projects, but he is equally engaged with (influencing) policy."
He seems to have few critics, although Professor Gorard said that, while he had huge respect for the Black Box research, it might not be that novel, simply spelling out the techniques good teachers had always used.
Robert Coe, a director of Durham university's curriculum evaluation and management centre, was tutored by Professor Wiliam on a PGCE course at King's College, London, in the 1980s.
Dr Coe said: "He was a great inspiration. He had a vision of the difference a teacher can make - that you can go into a rough school in London, for example, and improve things for pupils.
"I look back on my PGCE with quite fond memories. That's something you won't find many teachers saying, and he was part of that."
TALE OF A CLASSTEACHER'S JOURNEY TO THE TOP
* 1960-73: At school in Cardiff and at Altrincham grammar school, Trafford.
* 1973-76: Durham university. He also has an Open University BA, an MSc from the Polytechnic of the South Bank and a PhD from the University of London
* 1976-77: Tutor of maths and physics, St Cloud college, Worcestershire.
* 1977-80: Maths teacher at Christopher Wren school, London.
* 1980-84: Maths teacher at North Westminster school, London.
* 1984-86: Nuffield research fellow, Chelsea college, London.
* 1986-2003: Maths lecturer, subsequently senior lecturerin education, then dean, and assistant principal at King's college, London.
* 2003-06: Research director, Educational Testing Service, Princeton, New Jersey.
* 2006-: Deputy director, Institute of Education, London university