  We present an approach to the parametrized post-Newtonian (PPN) formalism
which is based on gauge-invariant higher order perturbation theory. This
approach divides the components of the metric perturbations into
gauge-invariant quantities, which carry information about the physical system
under consideration, and pure gauge quantities, which describe the choice of
the coordinate system. This separation generally leads to a simplification of
the PPN procedure, since only the gauge-invariant quantities appear in the
field equations and must be determined by solving them. Another simplification
arises from the fact that the gauge-invariant approach supersedes the necessity
to first choose a gauge for solving the gravitational field equations and later
transforming the obtained solution into the standard PPN gauge, as it is
conventionally done in the PPN formalism, whose standard PPN gauge is
determined only after the full solution is known. In addition to the usual
metric formulation, we also present a tetrad formulation of the gauge-invariant
PPN formalism. To illustrate their practical application, we demonstrate the
calculation of the PPN parameters of a well-known scalar-tensor class of
theories.
