  We prove the existence of a nontrivial solution (u \in H^1 (\R^N)) to the
nonlinear Choquard equation [- \Delta u + u = \bigl(I_\alpha \ast F (u)\bigr)
F' (u) \quad \text{in (\R^N),}] where (I_\alpha) is a Riesz potential, under
almost necessary conditions on the nonlinearity (F) in the spirit of Berestycki
and Lions. This solution is a groundstate; if moreover (F) is even and monotone
on ((0,\infty)), then (u) is of constant sign and radially symmetric.
