This book presents the essential aspects of relativistic quantum field theory, with minimal use of mathematics. It covers the development of quantum field theory from the original quantization of electromagnetic field to the gauge field theory of interactions among quarks and leptons. Aimed at both scientists and non-specialists, it requires only some rudimentary knowledge of the Lagrangian and Hamiltonian formulation of Newtonian mechanics and a basic understanding of the special theory of relativity and quantum mechanics.