In 1986 he was awarded the Fields Medal at the ICM at Berkeley for proving the Tate conjecture for abelian varieties over number fields, the Shafarevich conjecture for abelian varieties over number fields and the Mordell conjecture, which states that any non-singular projective curve of genus g > 1 defined over a number field K contains only finitely many K-rational points. As a Fields Medalist he gave an ICM plenary talk Recent progress in arithmetic algebraic geometry.
In 1994 as an ICM invited speaker in Zurich he gave a talk Mumford-Stabilität in der algebraischen Geometrie. Extending methods of Paul Vojta, he proved the Mordell–Lang conjecture, which is a generalization of the Mordell conjecture. Together with Gisbert Wüstholz, he reproved Roth's theorem, for which Roth had been awarded the Fields medal in 1958.