  In the last century the non-perturbative regularization of chiral fermions
was a long-standing problem. We review how this problem was finally overcome by
the formulation of a modified but exact form of chiral symmetry on the lattice.
This also provides a sound definition of the topological charge of lattice
gauge configurations. We illustrate a variety of applications to QCD in the p-,
the epsilon- and the delta-regime, where simulation results can now be related
to Random Matrix Theory and Chiral Perturbation Theory. The latter contains Low
Energy Constants as free parameters, and we comment on their evaluation from
first principles of QCD.
