Quantum field theory is the basic theory of elementary particle physics. In recent years, many techniques have been developed that extend and clarify this theory. This book develops quantum field theory starting from its foundation in quantum mechanics and incorporates the most modern methods, giving a thoroughly modern pedagogic account that starts from first principles. The path integral formulation is introduced right at the beginning. The method of dimensional continuation is employed to regulate and renormalize the theory. This facilitates the introduction of the concepts of the renormalization group at an early stage. The notion of spontaneous symmetry breakdown is also introduced early on by the example of superfluid helium. Topics in quantum electrodynamics are described that have an analog in quantum chromodynamics. Some novel techniques are employed, such as the use of dimensional continuation to compute the Lamb shift. This book is appropriate as a graduate level text in theoretical physics and particle physics, and is complete with exercises for practice.