  We review the O(a) improvement of lattice QCD with special emphasis on the
motivation for performing the improvement programme non-perturbatively and the
general concepts of on-shell improvement. The present status of the
calculations of various improvement coefficients (perturbative and
non-perturbative) is reviewed, as well as the computation of the isospin
current normalization constants $Z_A$ and $Z_V$. We comment on recent results
for hadronic observables obtained in the improved theory.
