  Within the framework of relativistic quantum field theory, a novel method is
established which allows to distinguish non-equilibrium states admitting
locally a thermodynamic interpretation. The basic idea is to compare these
states with global equilibrium states (KMS states) by means of local thermal
observables. With the help of such observables, the states can be ordered into
classes of increasing local thermal stability. Moreover, it is possible to
identify states exhibiting certain specific thermal properties of interest,
such as a definite local temperature or entropy density. The method is
illustrated in a simple model describing the spatio-temporal evolution of a
``big heat bang''.
