  We are concerned with the existence of ground states for nonlinear Choquard
equations involving a critical nonlinearity in the sense of
Hardy-Littlewood-Sobolev. Our result complements previous results by Moroz and
Van Schaftingen where the subcritical case was considered. Then, we focus on
the existence of semi-classical states and by using a truncation argument
approach, we establish the existence and concentration of single peak solutions
concentrating around minima of the Schr\"odinger potential, as the Planck
constant goes to zero. The result is robust in the sense that the nonlinearity
is not required to satisfy monotonicity conditions nor the
Ambrosetti-Rabinowitz condition.
