  A comparison of the potentials of ep and e^+e^-$machines to probe the
structure of the photon is performed. In particular, the kinematic reach of a
proposed future ep facility, THERA, is compared with those of current
colliders, LEP and HERA, and with the proposed linear collider, TESLA. THERA
like HERA will use a proton beam of 920 GeV but with an increased electron beam
energy of 250 GeV allowing higher scales, Q^2, and lower values of parton
momentum fraction in the photon, x_\gamma, to be probed.
