Integer solutions to polynomial equations — from Pythagorean triples to Fermat's Last Theorem (Wiles, 1995)
Proven by Andrew Wiles in 1995 using modularity of elliptic curves. The key insight: a solution would yield a Frey elliptic curve that cannot be modular, contradicting the Taniyama-Shimura-Weil conjecture. Pell's equation x²−Dy²=1 always has infinitely many solutions (fundamental solution via continued fractions).