  The powerful wind-wind collision in massive star binaries creates a region of
high temperature plasma and accelerates particles to relativistic energies. I
briefly summarize the hydrodynamics of the wind-wind interaction and the
observational evidence, including recent $\gamma$-ray detections, of
non-thermal emission from such systems. I then discuss existing models of the
non-thermal emission and their application to date, before concluding with some
future prospects.
