  There is growing evidence that some variable high-energy gamma-ray sources
are associated with pulsar wind nebulae. We review the current status of X-ray
and radio studies of the sources most likely to be significant emitters above
100 MeV. The assumption of the gamma-radiation arising from synchrotron
processes puts a lower limit on the magnetic field and suggests there may be
significant doppler boosting causing the asymmetries seen in the morphology of
these nebulae. However, these sources are also in close proximity to molecular
clouds, suggesting that variable gamma-ray emission may arise from a ram
pressure confined pulsar wind nebula in a dense medium.
