X-ray spectroscopy of active galactic nuclei.

X-ray spectroscopy of active galactic nuclei (AGN) has the potential for determining many of the fundamental properties of AGN. The author discusses the scientific goals of such observations with respect to the underlying energy source, the regions responsible for the optical emission lines, the different "types" of active galaxies and cosmology. The requirements for an X-ray mission of broad band width, large collecting area, modest spatial resolution and good spectral resolution are outlined. It is concluded that the proposed XMM mission meets these requirements.