  The structure of real and virtual photons has been studied in ep collisions
at HERA using dijet production. Measurements of differential dijet cross
sections as function of the fraction of photon's momentum invested in the dijet
system are presented. The dependence of the cross sections on the virtuality of
the photon and mean transverse energy squared of the two leading jets has been
investigated. QCD calculations based on current parametrizations of the real
and virtual photon parton distribution functions have been compared to the
data.
