In view of the fact that there is an abundance of excellent textbooks and monographs already on the market, it takes some special motivation of purpose, if not presumptuousness, to write yet another book on quantum theory. In the present case, the motivation was to fill a need of graduate students in theoretical physics that became apparent to me while teaching intermediate and advanced topics in quantum theory, during the last ten years or so, at universities in Europe, England, and the United States of America.