  X-ray transience is the most extreme form of variability observed in AGN or
normal non-active galaxies. While factors of 2-3 on timescales of days to years
are quite common among AGN, X-ray transients appear only once and vanish from
the X-ray sky years later. The ROSAT All-Sky Survey with its sensitivity to
energies down to 0.1 keV was the an ideal tool to discover these sources. X-ray
transience in AGN or galaxies can be caused by dramatic changes in the
accretion rate of the central black hole or by changes of the properties of the
accretion disk. So far only a handful of sources are known. In order to
estimate how often such an event occurs in a galaxy, a new soft X-ray survey is
needed. In these proceedings I describe the currently known X-ray transient AGN
and galaxies and will argue for a new soft X-ray survey in order to discover
more of these extreme X-ray sources.
