  We review some aspects of the fermionic interpretation of the two-dimensional
Ising model. The use is made of the notion of the integral over the
anticommuting Grassmann variables. For simple and more complicated 2D Ising
lattices, the partition function can be expressed as a fermionic Gaussian
integral. Equivalently, the 2D Ising model can be reformulated as a
free-fermion theory on a lattice. For regular lattices, the analytic solution
then readily follows by passing to the momentum space for fermions. We also
comment on the effective field-theoretical (continuum-limit) fermionic
formulations for the 2D Ising models near the critical point.
