Sir Anthony James Leggett (born 26 March 1938) is a British–American theoretical physicist and professor emeritus at the University of Illinois Urbana-Champaign (UIUC).

His mother's parents were of Irish descent; her father had moved to Britain and worked as a clerk in the naval dockyard in Chatham.

She eventually married his grandfather and raised a large family, then in her late sixties emigrated to Australia to join her daughter and son-in-law, and finally returned to the UK for her last years.

He was eventually followed by two sisters, Clare and Judith, and two brothers, Terence and Paul, all raised in their parents' Roman Catholic faith.

When he was 18 months old, WWII broke out and he was evacuated to Englefield Green, a small village in Surrey on the edge of the great park of Windsor Castle, where he stayed for the duration of the war.

After the end of the war, he returned to the Upper Norwood house and lived there until 1950; his father taught at a school in north-east London and his mother looked after the five children full-time.

Leggett won a scholarship to Balliol College, Oxford, in December 1954 and entered the University the following year with the intention of reading the degree technically known as Literae Humaniores (classics).

As with all of ter Haar's students in that period, the tentatively assigned thesis topic was "Some Problems in the Theory of Many-Body Systems", which left a considerable degree of latitude.

Dirk took a great interest in the personal welfare of his students and their families, and was meticulous in making sure they received adequate support; indeed, he encouraged Leggett to apply for a Prize Fellowship at Magdalen, which he held from 1963 to 1967.

In the end Leggett's thesis consisted of studies of two somewhat disconnected problems in the general area of liquid helium, one on higher-order phonon interaction processes in superfluid 4He and the other on the properties of dilute solutions of 4He in normal liquid 3He (a system which unfortunately turned out to be much less experimentally accessible than the other side of the phase diagram, dilute solutions of 3He in 4He).

Leggett's own research interests shifted away from superfluid 3He since around 1980; he worked inter alia on the low-temperature properties of glasses, high-temperature superconductivity, the Bose–Einstein condensate (BEC) atomic gases and above all on the theory of experiments to test whether the formation of quantum mechanics will continue to describe the physical world as we push it up from the atomic level towards that of everyday life.

The edition of 29 December 2005 of the International Herald Tribune printed an article, "New tests of Einstein's 'spooky' reality", which referred to Leggett's Autumn 2005 debate at a conference in Berkeley, California, with fellow Nobel laureate Norman Ramsey of Harvard University.