The underlying source of power in active galaxies and quasars is still debated. X-ray observations of these objects may relate more directly with this power source than observations in other wavebands. Theories of active galactic nuclei are reviewed, together with X-ray production mechanisms, both thermal and non-thermal. Constraints imposed on these theories by present and future observations are discussed and interpreted in terms of some models for specific objects.
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A. C. Fabian (1979) studied this question.