  Using the covariant approach and conformal transformations, we present a
gauge-invariant formalism for cosmological perturbations in generalized
Einstein theories (GETs), including the Brans-Dicke theory, theories with a
non-minimally coupled scalar field and certain curvature-squared theories. We
find an enhancement in the growth rate of density perturbations in the
Brans-Dicke theory, and discuss attractive features of GETs in the structure
formation process.
