Graduate students and researchers in astrophysics and cosmology need a solid grasp of a wide range of physical processes. This authoritative textbook helps readers develop the necessary toolkit of theory. The book is modular in design, allowing the reader to pick and chose a selection of chapters, if necessary. After reviewing the basics of dynamics, electromagnetic theory, and statistical physics, the book carefully develops a solid understanding of radiative processes, spectra, fluid mechanics, plasma physics and MHD, dynamics of gravitating systems, general relativity, nuclear physics, and other key concepts. Throughout, the reader's understanding is developed and tested with problems and helpful hints. This welcome volume provides graduate students with an indispensable introduction to and reference on all the physical processes they will need to successfully tackle cutting-edge research in astrophysics and cosmology. It can be used alone or in conjunction with two companion volumes, which cover stars and stellar systems, and galaxies and cosmology (both forthcoming).