  Previous generations of X-ray observatories revealed a group of massive
binaries that were relatively bright X-ray emitters. This was attributed to
emission of shock-heated plasma in the wind-wind interaction zone located
between the stars. With the advent of the current generation of X-ray
observatories, the phenomenon could be studied in much more detail. In this
review, we highlight the progress that has been achieved in our understanding
of the phenomenon over the last 15 years, both on theoretical and observational
grounds. All these studies have paved the way for future investigations using
the next generation of X-ray satellites that will provide crucial information
on the X-ray emission formed in the innermost part of the wind-wind
interaction.
