  The N=1 effective action for generic type IIA Calabi-Yau orientifolds in the
presence of background fluxes is computed from a Kaluza-Klein reduction. The
Kahler potential, the gauge kinetic functions and the flux-induced
superpotential are determined in terms of geometrical data of the Calabi-Yau
orientifold and the background fluxes. The moduli space is found to be a Kahler
subspace of the N=2 moduli space and shown to coincide with the moduli space
arising in compactification of M-theory on a specific class of G_2 manifolds.
The superpotential depends on all geometrical moduli and vanishes at leading
order when background fluxes are turned off. The N=1 chiral coordinates
linearize the appropriate instanton actions such that instanton effects can
lead to holomorphic corrections of the superpotential. Mirror symmetry between
type IIA and type IIB orientifolds is shown to hold at the level of the
effective action in the large volume - large complex structure limit.
