This text develops quantum theory from its basic assumptions, beginning with statics, followed by dynamics and details of applications and the needed computational techniques. The discussion is based on the view that the fundamental entities of the universe are not particles but fields, with the observed particles arising as their quanta. Quantum fields are thus introduced from the beginning, with a discussion of how they produce quanta that manifest themselves as particles. Most of the book, of course, deals with particle systems, as that is where most of the applications lie; the treatment of quantum field theory is confined to fundamental ideas and their consequences. For developing quantum dynamics, the author uses the Lagrangian technique with the principle of stationary action. The roots of this approach, which includes generating the canonical commutation rules, go back to a course taught by Julian Schwinger, filtered through many years of the author's own teaching.