  'Constituent quarks' means massive quarks, in contrast to the nearly massless
u,d,s quarks of the QCD lagrangian. The dynamical or constituent masses appear
owing to the spontaneous chiral symmetry breaking in QCD. A realistic mechanism
of the chiral symmetry breaking is provided by instantons. Therefore, I review
the present status of the QCD instanton vacuum, emphasizing the mechanism of
chiral symmetry breaking and of the generation of the constitutent quark mass.
I end up with the Nambu--Jona-Lasinio-type effective theory to which QCD is
reduced at low momenta.
