  A new approach to quantum field theory at finite temperature and density in
arbitrary space-time dimension D is developed. We focus mainly on relativistic
theories, but the approach applies to non-relativistic ones as well.
  In this quasi-particle re-summation, the free energy takes the free-field
form but with the one-particle energy $\omega (\vec{k})$ replaced by $\vep
(\vec{k})$, the latter satisfying a temperature-dependent integral equation
with kernel related to a zero temperature form-factor of the trace of
stress-energy tensor. For 2D integrable theories the approach reduces to the
thermodynamic Bethe ansatz. For relativistic theories, a thermal c-function
$C_{\rm qs} (T)$ is defined for any $D$ based on the coefficient of the black
body radiation formula. Thermodynamical constraints on it's flow are presented,
showing that it can violate a ``c-theorem'' even in 2D. At a fixed point
$C_{\rm qs}$ is a function of thermal gap parameters which generalizes Roger's
dilogarithm to higher dimensions. This points to a strategy for classifying
rational theories based on ``polylogarithmic ladders'' in mathematics, and many
examples are worked out. An argument suggests that the 3D Ising model has
$C_{\rm qs} = 7/8$. (In 3D a free fermion has $C_{\rm qs} = 3/4$.) Other
applications are discussed, including the free energy of anyons in 2D and 3D,
phase transitions with a chemical potential, and the equation of state for
cosmological dark energy.
