In this text the authors develop a propagator theory of Dirac particles, photons, and Klein-Gordon mesons and per- form a series of calculations designed to illustrate various useful techniques and concepts in electromagnetic, weak, and strong interactions. these include defining and implementing the renormalization program and evaluating effects of radia- tive corrections, such as the Lamb shift, in low-order calculations. The necessary background for the book is pro- vided by a course in nonrelativistic quantum mechanics at the general level of Schiff's text, QUANTUM MECHANICS.