  We study the equivalence principle and its violations by quantum effects in
scalar-tensor theories that admit a conformal frame in which matter only
couples to the spacetime metric. These theories possess Ward identities that
guarantee the validity of the weak equivalence principle to all orders in the
matter coupling constants. These Ward identities originate from a broken Weyl
symmetry under which the scalar field transforms by a shift, and from the
symmetry required to couple a massless spin two particle to matter
(diffeomorphism invariance). But the same identities also predict violations of
the weak equivalence principle relatively suppressed by at least two powers of
the gravitational couplings, and imply that quantum corrections do not preserve
the structure of the action of these theories. We illustrate our analysis with
a set of specific examples for spin zero and spin half matter fields that show
why matter couplings do respect the equivalence principle, and how the
couplings to the gravitational scalar lead to the weak equivalence principle
violations predicted by the Ward identities.
