  In this article aspects of photon-photon physics related to the structure of
real and virtual photons are reviewed. A re-calculation of the virtual
photon-photon box is performed and some discrepancies in the literature are
clarified. A useful compilation of various relevant limits derived from the
most general expressions is provided. Furthermore, structure functions of
spin-averaged, transverse and longitudinal virtual target photons are defined
and discussed. Finally, the factorization of two-photon processes in the
Bjorken limit is demonstrated to hold also for the case of virtual target
photons.
