He works on special values of L-functions and -adic representations of -adic groups at the meeting point of Fontaine's and Langlands' programs. His contributions include:
A conjecture: the Colmez conjecture relating Artin L-functions at and periods of abelian varieties with complex multiplication, a far-reaching generalization of the Chowla-Selberg formula.
A proof of Perrin-Riou's conjectural explicit reciprocity law related to the functional equation of -adic L-functions.
Several contributions to Fontaine's program of classification of -adic representations of the absolute Galois group of a finite extension of , including proofs of conjectures of Fontaine such as "weakly admissible implies admissible" and the "-adic monodromy conjecture" which describe representations coming from geometry, or the overconvergence of all representations, and the addition of new concepts such as "trianguline representations" or "Banach-Colmez spaces".
A construction of the -adic local Langlands correspondence for , via the construction of a functor (known as "Colmez's functor" or "Colmez's Montreal functor") from representation of to representations of the absolute Galois group of .
Comparison theorems for -adic algebraic and analytic varieties with applications to a geometrization of the -adic local Langlands correspondence.
With Jean-Pierre Serre, he co-edited the Correspondance Grothendieck-Serre (2001) and the Correspondance Serre-Tate (2015).
↑Cohomologie -adique de la tour de Drinfeld, le cas de la dimension 1, Journal of the AMS 33 (2020), 311–362 (with Wiesława Nizioł and Gabriel Dospinescu).
↑Cohomology of -adic Stein spaces, Inventiones mathematicae 219 (2020), 873–985 (with Wiesława Nizioł and Gabriel Dospinescu).