Description More than thirty years later, instantons continue to be of interest and the timely republication of this classic paper provides an accessible reference text for students and researchers. Recently, L. F. Alday, D. Gaiotto and Y. Tachikawa proposed the striking relation between 2-conformal field theory and N=2 SUSY d=4QCD confirms the current increasing interest in instantons and highlights the importance and contribution of the fundamental work