International Congress of Mathematicians (ICM)
The International Congress of Mathematicians has been held every four years (nominally  there have been interruptions due to war and other causes) since the first Congress held in Chicago in 1893 in honor of Columbus' voyage to America. The most famous mathematical address of all time was Hilbert's keynote address to the 1900 congress in Paris; his talk consisted of a discussion of 23 unsolved problems, now called Hilbert's problems. They were chosen as problems whose solution would
require the invention of new techniqes, which in turn would lead to the creation of new areas of mathematics. To date, slightly over half of them have been solved. Since 1936, one of the highlights of each congress has been the announcement and awarding of the Fields medals, the closest mathematical approximation to a Nobel Prize; one unique feature of the Fields medal is that it is awarded only to mathematicians for work done while under the age of 40. There are many colorful stories as to why there is no Nobel Prize in mathematics; most involve a supposed fear on Nobel's part that the prize might someday be awarded to Mittagleffler (some stories include mention of a relationship between MittagLeffler and Nobel's wife; these are false, for Nobel never married), but the truth seems to be that Nobel just wasn't interested in mathematics. No Fields medalist has ever appeared on a stamp, but host countries have issued stamps featuring the ICM on four occasions; the United States did not do so for the 1986 congress held in Berkeley. Three appear below. The Japanese stamp was given to me by a friend who attended the Congress.
International Congress of Mathematicians 
International Congress of Mathematicians 
Finland (1978), No. 612 
Japan (1990), No. 2060 




International Congress of Mathematicians 

Russia (1966), No. 3151 



I also have a first day cover for the Finnish stamp.
International Congress of Mathematicians 
Finland (1978), No. 612 
Poland issued a series of four stamps for the 1982 Congress (actually held in 1983) featuring the Polish mathematicians Zaremba, Sierpinski, Janiszewski, and Banach, all of whom were instrumental in the creation of the Polish School of Mathematics. Here are the Polish stamps.
Stanslaw Zaremba (18631942) 
Waclaw Sierpinski (18821969) 
Poland (1982), No. 2542 
Poland (1982), No. 2543 


Zygmunt Janiszewski (18881920) 
Stefan Banach (18921945) 
Poland (1982), No. 2544 
Poland (1982), No. 2545 