Observations of stellar wind instabilities and variability on small and large scales

I review various observations which suggest that the winds of hot stars are inhomogeneous because of instabilities in the wind flow. On large scales, local wind overdensities are indirectly detected in the form of excess in t he infra-red (IR) and radio free-free continuum. The X-ray detection of a h ot (T similar to 10(6)) wind component suggests that the wind is pervaded w ith strong shocks. The small-scale density structure of the wind can be stu died from observations of Line-Profile Variations (LPVs) in optical and UV spectral lines, which are formed close to the stellar surface. LPVs in lines of the P Cygni type consist of blue-edge variations in satura ted profiles, and Discrete Absorption Components (DACs) and Periodic Absorp tion Modulations (PAMs) in unsaturated profiles. These LPVs are shown to be recurrent, and thought to result from instabilities propagating through th e wind and generated at the stellar surface. LPVs in recombination lines ap pear as stochastic subpeaks, which suggest that wind instabilities have a c lump-like, rather than shell-like, structure. The kinematics of LPVs in bot h line types is consistent with wind propagating shocks generated from radi ative instabilities.