Over 500 galaxies with active nuclei (AGN), in which class are included Seyfert galaxies, radio galaxies, QSOs and BL Lacertid-type objects, have so far been detected at X-ray wavelengths. Most of these are faint objects for which the only information available is a flux obtained from an observation with the Einstein Observatory, but somewhere upwards of 50 AGN are bright enough that it has been possible to obtain spectral and timing data on them. The timing observations reveal variability on all scales from minutes (and possibly shorter) to years whilst the spectra are generally best-fitted by power-laws, with some lower luminosity AGN showing evidence of absorption by cold gas at low X-ray energies. The X-ray spectral and timing observations, coupled with observations in other bands such as those of broad optical emission lines, have led to a generally accepted model for AGN whereby the primary power source is conversion of gravitational potential energy into luminous (heat) energy by accretion of cool matter onto a central massive black hole. An accretion rate of ~ 3M⊙ a year onto a 108 – 109 M⊙ black hole is sufficient to produce the observed luminosity in a source as powerful as the QSO 3C273 (see Rees 1984 for a detailed review of black hole models). A very hot plasma surrounding the black hole is thereby produced and determining the mechanisms by which this plasma radiates, whether they be thermal or non-thermal and whether relativistic beaming of the radiation is involved, is one of the major aims of AGN research.