A concise yet comprehensive introduction to the central theoretical concepts of modern astrophysics, presenting hydrodynamics, radiation, and stellar dynamics all in one textbook. Adopting a modular structure, the author illustrates a small number of fundamental physical methods and principles, which are sufficient to describe and understand a wide range of seemingly very diverse astrophysical phenomena and processes. For example, the formulae that define the macroscopic behavior of stellar systems are all derived in the same way from the microscopic distribution function. This function itself may be obtained from fundamental kinetic theory, and it is a discussion of these relations that constitutes the second chapter of this book. The concept of distribution functions and Boltzmann's equation is defined, while the equilibrium distribution function for ideal gases is calculated, followed by a discussion of small perturbations and their relaxation back to the equilibrium state. Finally, the author shows that macroscopic quantities are usually associated with moments of the distribution function and derive their evolutionary equations. Aimed at students at graduate level and lecturers teaching courses in theoretical astrophysics or advanced topics in modern astronomy, this book with its abundant examples and exercises also serves as a reference and entry point for more advanced researchers wanting to update their knowledge of the physical processes that govern the behavior and evolution of astronomical objects.