  We use perturbation theory to construct perfect lattice actions for fermions
and gauge fields by blocking directly from the continuum. When one uses a
renormalization group transformation that preserves chiral symmetry the
resulting lattice action for massless fermions is chirally symmetric but
nonlocal. When the renormalization group transformation breaks chiral symmetry,
the lattice action becomes local but chiral symmetry is explicitly broken. In
particular, starting with a chiral gauge theory in the continuum one either
obtains a lattice theory which is gauge invariant but nonlocal, or a local
theory with explicitly broken gauge invariance. In both cases the spectrum of
the lattice theory is identical with the one of the continuum and the anomaly
is correctly reproduced. We also apply our techniques to vector-like theories.
In particular we propose a new renormalization group transformation for QCD and
we optimize its parameters for locality of the perfect action.
