Recent results relating to the X-ray variability exhibited by active galactic nuclei are reviewed. EXOSAT observations have established, contrary to earlier indications, that X-ray variability on a timescale of an hour or less is a relatively common feature of such sources. It is argued that the minimum variability timescale may be a direct indicator of the mass of the black hole in an active galactic nucleus. In some active galaxies the central black hole mass may be ≤107 M⊙.