  The lower order terms of the heat kernel expansion at coincident points are
computed in the context of finite temperature quantum field theory for flat
space-time and in the presence of general gauge and scalar fields which may be
non Abelian and non stationary. The computation is carried out in the imaginary
time formalism and the result is fully consistent with invariance under
topologically large and small gauge transformations. The Polyakov loop is shown
to play a fundamental role.
