  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..
