  We investigate the low-energy behavior of the four-point Green's function
$\Gamma^{\mu\nu}$ describing virtual Compton scattering off the nucleon.
  Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive
the leading terms of an expansion of the operator in the four-momenta $q$ and
$q'$ of the initial and final photon, respectively.
  The model-independent result is expressed in terms of the electromagnetic
form factors of the free nucleon, i.e., on-shell information which one obtains
from electron-nucleon scattering experiments.
  Model-dependent terms appear in the operator at $O(q_\alpha q'_\beta)$,
whereas the orders $O(q_\alpha q_\beta)$ and $O(q'_\alpha q'_\beta)$ are
contained in the low-energy theorem for $\Gamma^{\mu\nu}$, i.e., no new
parameters appear.
  We discuss the leading terms of the matrix element and comment on the use of
on-shell equivalent electromagnetic vertices in the calculation of ``Born
terms'' for virtual Compton scattering.
