  We review problems involving the use of Grassmann techniques in the field of
classical spin systems in two dimensions. These techniques are useful to
perform exact correspondences between classical spin Hamiltonians and
field-theory fermionic actions. This contributes to a better understanding of
critical behavior of these models in term of non-quadratic effective actions
which can been seen as an extension of the free fermion Ising model. Within
this method, identification of bare masses allows for an accurate estimation of
critical points or lines and which is supported by Monte-Carlo results and
diagrammatic techniques.
