Fermat's "biggest", and also his "last" theorem states that xn + yn = zn has no solutions in positive integers x, y, z with n greater than 2. This has finally been proven by Wiles in 1995. However, in the study of primes it is Fermat's little theorem that is most used:
It is so easy to calculate ap-1 that most elementary primality tests are built using a version of Fermat's Little Theorem rather than Wilson's Theorem.
As usual, Fermat did not provide a proof (this time saying "I would send you the demonstration, if I did not fear its being too long" [Burton80, p79]). Euler first published a proof in 1736, but Leibniz left virtually the same proof in an unpublished manuscript from sometime before 1683.
See Also: FermatQuotient
Related pages (outside of this work)