  Thermal field theory is reviewed briefly. It is noted that, until recently,
it was not known what type of real-time Green function is being calculated in
the Euclidean approach. The formal answer to this question is then given and
the unexpectedly complicated answer is discussed. The physical implications of
the results are then considered. (Talk given at the Lake Louise Winter
Institute on Particle Physics and Cosmology, 20th - 26th February, 1994, Lake
Louise, Canada)
