  We give a pedagogical review of implications of chiral symmetry in QCD.
First, we briefly discuss classical textbook subjects such as the axial
anomaly, spontaneous breaking of the flavor-nonsinglet chiral symmetry,
formation of light pseudo-Goldstone particles, and their effective
interactions. Then we proceed to other issues. We explain in some detail a
recent discovery how to circumvent the Nielsen--Ninomiya's theorem and
implement chirally symmetric fermions on the lattice. We touch upon such
classical issues as the Vafa-Witten's theorem and 't Hooft's anomaly matching
conditions. We derive a set of exact theorems concerning the dynamics of the
theory in a finite Euclidean volume and the behavior of the Dirac spectral
density. Finally, we discuss an imaginary world with a nonzero value of the
vacuum angle theta.
