“…This was achieved, by Bogoliubov, Parasiuk, Hepp, Zimmermann, Epstein, Glaser, Steinmann and others, in a twenty years struggle, and the finally reached state of the art is nicely documented in the proceedings of the Erice school 1975 dedicated to renormalization [VW76]. Main highlights are the Forest Formula of Zimmermann [Zim69] which solves the recursion relations of the Bogoliubov-Parasiuk-Hepp (BPH) method [BP57,BS59,Hep66], the causal method of Epstein-Glaser (EG) [EG73], elaborating on older attempts of Stückelberg [SR50] and Bogoliubov [BP57,BS59], and the method of retarded products by Steinmann [Ste71].…”