This review differs very much from other reviews on the subject. Instead of giving a detailed picture of accretion that follows from microscopic properties of matter (“dirty astrophysics”) I summarize a few fundamental principles on which the theory of black hole accretion disks rests. I stress that although the detailed properties of accretion disks modelsof coursedepend very much on dirty astrophysics, the phenomenon of black hole accretion disks is very simple indeed. ©1998 American Institute of Physics.