Geometric methods for cohomological invariants

We explain how to exploit Rost's theory of Chow groups with coefficients to carry some computations of cohomological invariants (i.e. characteristic classes for G-torsors in Galois cohomology, for an algebraic group G). In particular, we use the idea of the ``stratification method'' introduced by Vezzosi. We recover a number of known results, with very different proofs. We obtain some new information on spin groups..