This textbook covers elements of quantum field theory, symmetry principles, gauge field theories and phenomenological descriptions of hadrons, with special emphasis on topics relevant to hadron and nuclear physics. Written at an introductory level, it is aimed at nuclear physicists in general and experimentalists in particular who need a working knowledge of field theory, symmetry principles of elementary particles and their interactions and the quark structure of hadrons. It will also be of benefit to graduate students who need an understanding of the basics of these topics for their work in other fields.