Radio observations of colliding winds in massive stars

This brief review describes radio observations of colliding winds in massive stars starting with the first direct observational support for the colliding-wind model advanced in the early 1990’s to explain non-thermal radio and thermal X-ray emission in some massive stars. Studies of the well-studied and highly-eccentric WR+O star system WR140 are described along with recent observations of O-star systems. Also discussed is the binary nature of almost all massive stars that exhibit non-thermal behavior and some strategies for finding new systems.