Twenty years of lectures on the quantum world by an esteemed physicist. This book covers the material of the two-semester quantum field theory course that Jir� Horejs� has taught at Charles University and Czech Technical University in Prague for over two decades. In the individual chapters, one may find the discussion of selected topics in relativistic quantum mechanics and relativistic quantum field theory; the dominant theme is quantum electrodynamics. The technique of Feynman diagrams is described in detail, along with methods of regularization and renormalization, including some basic applications. The selection of topics presented in the book is intended to provide the reader with the technical skills necessary for the subsequent study of theoretical particle physics. In keeping with the author's typical lecture style, the text contains many detailed explicit calculations to a degree not entirely typical in other available sources. With primary appeal for university students specializing in theoretical physics or nuclear and particle physics, it may also be useful for any scientifically minded reader seriously interested in the foundations of modern physics.