  Given a p-adic representation of the Galois group of a local field, we show
that its Galois cohomology can be computed using the associated etale
(phi,Gamma)-module over the Robba ring; this is a variant of a result of Herr.
We then establish analogues, for not necessarily etale (phi,Gamma)-modules over
the Robba ring, of the Euler-Poincare characteristic formula and Tate local
duality for p-adic representations. These results are expected to intervene in
the duality theory for Selmer groups associated to de Rham representations.
