Book excerpt: Introduction to Elementary Particle Theory details the fundamental concepts and basic principles of the theory of elementary particles. The title emphasizes on the phenomenological foundations of relativistic theory and to the strong interactions from the S-matrix standpoint. The text first covers the basic description of elementary particles, and then proceeds to tackling relativistic quantum mechanics and kinematics. Next the selection deals with the problem of internal symmetry. In the last part, the title details the elements of dynamical theory. The book will be of great use to students and researchers in the field of particle physics.