  We present results of a global tuning of general purpose Monte Carlo models
to published measurements of photon-proton -> jets at HERA and photon-photon ->
jets at LEP and TRISTAN. The principle free parameters in the tuning are the
simulation of the underlying event and the choice of photon structure. Several
combinations of models are ruled out by the data. Some consequences of the
tuned models at a future linear collider are discussed.
