All main sequence stars lose mass via stellar winds. The winds of cool stars like the sun are 
driven by gas pressure gradient. However, the winds of hot massive stars which tend to be luminous 
are driven by emitted by the star radiation pressure.
In this talk, I  describe the nature of such radiative driving and show that the continuum and line 
opacities in the wind determine how large the stellar atmosphere may appear in an interferometer.
Currently, interferometers can detect only relatively large scale structures. I will describe how 
these structures are induced by rotation, pulsations, magnetic fields or wind-wind interactions.