The Fields Medal is awarded every four years during the opening ceremony of the International Congress of Mathematicians (ICM). It recognizes outstanding mathematical achievement for existing work and for the promise of future achievement. Two to four medals are awarded to mathematicians who have to be of age less than forty years on January 1 of the Congress year. The Fields Medal, established in 1936 and named after the Canadian mathematician J. C. Fields, is one of the most prestigious awards in the field of mathematics and often described as the “Nobel Prize of Mathematics”.
Further information: http://www.mathunion.org/general/prizes/fields/details