  A procedure is proposed to study QFT at zero or at finite temperature by a
sequence of real space renormalization group (RG) or blockspin transformations.
They transform to effective theories on coarser and coarser lattices.The
ultimate aim is to compute constraint effective potentials, i.e. the free
energy as a function of suitable order parameters.From the free energy one can
read off the thermodynamic behaviour of the theory, in particular the existence
and nature of phase transitions. In a finite temperature QFT one begins with
either one or a sequence of transformations which transform the original theory
into an effective theory on a three-dimensional lattice. Its effective action
has temperature dependent coefficients. Thereafter one may proceed with further
blockspin transformations of the three-dimensional theory. Assuming a finite
volume, this can in principle be continued until one ends with a lattice with a
single site. Its effective action is the constraint effective potential. In
each RG-step, an integral over the high frequency part of the field has to be
performed. This is done perturbatively and requires the knowledge of bare
fluctuation field propagators and of interpolation operators which enter into
the vertices. A detailed examination of these quantities is presented for
scalar fields, abelian gauge fields and for Higgs fields, finite temperature is
admitted.
