At the 1924 International Congress of Mathematicians in Toronto,
a resolution was adopted that at each ICM, two gold medals should
be awarded to recognize outstanding mathematical achievement.
Professor J. D. Fields, a Canadian mathematician who was secretary
of the 1924 Congress, later donated funds establishing the medals
which were named in his honor. Consistent with Fields's wish that
the awards recognize both existing work and the promise of future
achievement, it was agreed to restrict the medals to mathematicians
not over forty at the year of the Congress. In 1966 it was agreed
that, in light of the great expansion of mathematical research,
up to four medals could be awarded at each Congress.
For more details about the origins of the Fields Medal we recommend
Henry S. Tropp, "The Origins and History
of the Fields Medal", Historia Mathematica 3 (1976)
The following text by Eberhard Knobloch describes the design
of the medal: