  A brief introduction is given to the concept of the effective average action.
Its dependence on the averaging or coarse graining scale is governed by an
exact RG equation for which nonperturbative approximation schemes are
described. This formalism is applied to the computation of the equation of
state for two flavor QCD within an effective linear quark meson model. Our
results allow to derive the temperature and quark mass dependence of quantities
like the chiral condensate or the pion mass. A precision estimate of the
universal critical equation of state for the three--dimensional O(4) Heisenberg
model is given. The exact RG formalism applied to the linear quark meson model
is demonstrated to provide an explicit link between the O(4) universal behavior
near the critical temperature and zero quark mass on one hand and the physics
at low temperatures and realistic current quark masses, i.e., the domain of
validity of chiral perturbation theory on the other hand.
