We discuss the low-energy behaviour of the virtual Compton scattering amplitude off a spin-zero target. We first compare various methods of obtaining a low-energy expression based either on the soft-photon approximation or the use of Ward-Takahashi identities. We point out that structure-dependent terms are defined with respect to a low-energy approximation of the pole terms, which commonly is separated from the full amplitude. We derive a general expression for the structure-dependent terms in an expansion in terms of the momenta k_1 and k_2 of the initial and final virtual photon, respectively, up to and including terms of order O(k~4). At order O(k~2) two terms appear that are related to the usual electric and magnetic polarizabilities of real Compton scattering. At order O(k~4) we find nine new structures of which five can only be determined using virtual photons.
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