The BPST instanton (Belavin-Polyakov-Schwarz-Tyupkin 1975) is a self-dual solution of Euclidean SU(2) Yang-Mills: F=*F. It has topological charge Q=1, saturating |S|≥8π²|Q|/g². Instantons mediate tunneling between |n⟩ vacua, breaking U(1)_A symmetry.