Focusing on close binary systems containing compact objects and on active nuclei, the authors describe the physical processes at work in accretion discs and other accretion flows, beginning with why accretion is a source of energy. They then present the gas dynamics and plasma concepts necessary for astrophysical applications. Accretion in stellar systems is then covered, including accretion onto compact objects. Further chapters treat accretion in active galactic nuclei and describe thick accretion discs. A new chapter discusses recently discovered accretion flow solutions. Updated and expanded, this edition includes new material on disc instabilities, irradiated discs, disc warping, and general accretion flows. Suitable for advanced undergraduates, graduate students, and researchers.