This book provides an account of the foundations of quantum field theory, especially the Euclidean formulation. A simple (and rigorous) exposition of the problem concerning the general (nonperturbative) structures of local (renormalizable) formulations of gauge QFT and the general strategy for its solution is discused in a language addressed also to elementary particle theoreticians. It is important to understand the basic mechanisms of confinement, non-trivial topology and *vacua, chiral symmetry breaking which are expected to characterize QCD. This book also aims to settle unclear or debated points on this subject using a treatment close to the conventional local and covariant approach to gauge QFT.