  We study the definition of trace anomalies for models of Dirac and Weyl
fermions coupled to a metric and a gauge potential. While in the
non-perturbative case the trace anomaly is the response of the effective action
to a Weyl transformation, the definition in a perturbative approach is more
involved. In the latter case we use a specific formula proposed by M.Duff, of
which we present a physical interpretation. The main body of the paper consists
in deriving trace anomalies with the above formula and comparing them with the
corresponding non-perturbative results. We show that they coincide and stress
the basic role of diffeomorphism invariance for the validity of the approach.
