  The dominant cutoff effects in lattice QCD with Wilson quarks are
proportional to the lattice spacing a. In particular, the isovector axial
current satisfies the PCAC relation only up to such effects. Following a
suggestion of Symanzik, they can be cancelled by adding local O(a) correction
terms to the action and the axial current. We here address a number of
theoretical issues in connection with the O(a) improvement of lattice QCD and
then show that chiral symmetry can be used to fix the coefficients multiplying
the correction terms.
