  Very massive stars occasionally expel material in colossal eruptions, driven
by continuum radiation pressure rather than blast waves. Some of them rival
supernovae in total radiative output, and the mass loss is crucial for
subsequent evolution. Some are supernova impostors, including SN precursor
outbursts, while others are true SN events shrouded by material that was
ejected earlier. Luminous Blue Variable stars (LBV's) are traditionally cited
in relation with giant eruptions, though this connection is not well
established. After four decades of research, the fundamental causes of giant
eruptions and LBV events remain elusive. This review outlines the basic
relevant physics, with a brief summary of essential observational facts.
Reasons are described for the spectrum and emergent radiation temperature of an
opaque outflow. Many proposed mechanisms are noted for instabilities in the
star's photosphere, in its iron opacity peak zones, and in its central region.
Some of the remarks and conjectures here have not yet become familiar in the
published literature.
