Treating chiral symmetry in standard lattice QCD(quantum chromodynamics) approach has been a difficult issue. Due this difficulty, researchers often did extrapolations from un-physically massive u and d quarks. As a way for a better treatment of chiral symmetry, Brower, Shen and Tan proposed "chirally extended QCD" (also called XQCD) in 1994. An important basic scheme in XQCD is to add "irrelevant" terms to the action so that we have chiral symmetry on the lattice while having desired continuum limit. This book introduces part of the recent research of XQCD by R. Brower, C-I Tan, R. Edwards,...