Quantum Theory, comprising the three volumes, Elements, Aggregates, and Radiation and High Energy Physics is intended as an advanced text and reference on the fundamentals and applications of quantum theory.It is primarily designed to meet the needs of postgraduate students. The hope is that it will enable them to refresh and deepen their understanding of the elementary parts of the subject, that it will provide them with surveys of the more important areas of interest, and that it will guide them to the main frontiers on which advances are being made. In addition, teachers at universities and institutes of technology may find the compilation, which is unusually wide in scope, useful when preparing lectures.