  This review describes the evidence for small-scale structure, `clumping', in
the radiation line-driven winds of hot, massive stars. In particular, we focus
on examining to what extent simulations of the strong instability inherent to
line-driving can explain the multitude of observational evidence for wind
clumping, as well as on how to properly account for extensive structures in
density and velocity when interpreting the various wind diagnostics used to
derive mass-loss rates.
