  Interactions of virtual photons are analyzed in terms of photon structure. It
is argued that the concept of parton distribution functions is
phenomenologically very useful even for highly virtual photons involved in hard
collisions. This claim is illustrated on leading order expressions for
$F_2^{\gamma}(x,P^2,Q^2)$ and effective parton distribution function
$D_{\mathrm eff}(x,P^2,Q^2)$ relevant for jet production, as well as within the
next-to-leading order QCD calculations of jet cross-sections in electron-proton
collisions.
