This book provides an overview on transport theories, focusing on applications and the relativistic off-shell transport theory which are of particular interest for physicists working in the field of relativistic strong-interaction physics, e.g. relativistic or ultra-relativistic heavy-ion collisions or the evolution of the early universe. In this regard, a thorough derivation of the transport equations and a careful analysis of the approximations employed is given. The text is enriched with a multitude of Appendices that partly recall elements of quantum mechanics and field theory or present examples for specific models. Specific exercises are given throughout the chapters. As a basic knowledge the reader should be familiar with quantum mechanics and its principles as well as some basic concepts of the quantum many-body physics and field theory. All chapters close with a short summary and numerical calculations are provided to master and illustrate the subject.