  This article first reviews the calculation of the N = 1 effective action for
generic type IIA and type IIB Calabi-Yau orientifolds in the presence of
background fluxes by using 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. As a new result, it is shown that the chiral description directly
relates to Hitchin's generalized geometry encoded by special odd and even forms
on a threefold, whereas a dual formulation with several linear multiplets makes
contact to the underlying N = 2 special geometry. In type IIB setups, the
flux-potentials can be expressed in terms of superpotentials, D-terms and,
generically, a massive linear multiplet. The type IIA superpotential depends on
all geometric moduli of the theory. It is reviewed, how type IIA orientifolds
arise as a special limit of M-theory compactified on specific G_2 manifolds by
matching the effective actions. In a similar spirit type IIB orientifolds are
shown to descend from F-theory on a specific class of Calabi-Yau fourfolds. In
addition, mirror symmetry for Calabi-Yau orientifolds is briefly discussed and
it is shown that the N = 1 chiral coordinates linearize the appropriate
instanton actions.
