This two-volume book series is an attempt to formulate a consistent relativistic quantum theory of interacting charged particles. The major construction will be undertaken in the second volume. Our goal in this first volume is to prepare the reader for that effort. Here we will introduce notation, terminology, and basic ideas of relativistic quantum theories. Our discussion will proceed systematically and, more or less traditionally, from the principle of relativity and postulates of measurements to the renormalization in quantum electrodynamics. This will cover material usually found in textbooks on quantum field theory. However, we will choose particles (rather than fields) as our basic ingredients. Quantum fields will be regarded as mere mathematical tools, which are convenient for building relativistically invariant and cluster separable particle interactions. The reader is expected to have a good grasp of non-relativistic quantum mechanics.