  It is well-known that thermal equilibrium states in quantum statistical
mechanics and quantum field theory can be described in a mathematically
rigorous manner by means of the so-called Kubo-Martin-Schwinger (KMS)
condition, which is based on certain analyticity and periodicity properties of
correlation functions. On the other hand, the characterization of
non-equilibrium states which only locally have thermal properties still
constitutes a challenge in quantum field theory. We discuss a recent proposal
for characterization of such states by a generalized KMS condition. The
connection of this proposal to a proposal by D. Buchholz, I. Ojima and H.-J.
Roos for characterizing local thermal equilibrium states in quantum field
theory is discussed.
