Chiral symmetry is exhibited by vector gauge theories with massless fermions. Since there is no known massless fermion in nature, chiral symmetry is at best an approximate symmetry in fundamental theories, and the axial current is not conserved. (Note: this explicit breaking of the chiral symmetry by non-zero masses is not to be confused with the spontaneous chiral symmetry breaking that plays a dominant role in hadronic physics.) An important consequence of such non-conservation is the neutral pion decay and the chiral anomaly, which is directly related to the pion decay width.