A. H. Halsey has been a professorial fellow (now emeritus) at Nuffield College in Oxford University since his appointment in 1962 as Director of Oxford's Department of Social and Administrative Studies. This paper explores his contribution to education throughout his career, as an academic and as a national and international policy advisor, and the interface between these two. Halsey worked in what he termed the "political arithmetic" tradition throughout his career, with the dual tasks of documenting the state of society, and addressing social and political issues through "experimental social administration", that is the field testing of social innovation and social policy in advance of national implementation. The paper focuses on Halsey's "activist" role in policy development in the UK and internationally, through his work on educational reform at the OECD and as research advisor to Crosland at the DES with the introduction of comprehensive schooling in the UK and in particular the Educational Priority Areas (EPA) programme, and traces through the impact of his work. His major contribution as one of the leading sociologists of education in the second half of the 20th century is also discussed, but the wider impact of this aspect of his work requires a much more extensive assessment than is possible in this paper. (Contains 43 notes.)