  It is observationally as well as theoretically well established that the
winds of hot, massive OB-stars are highly structured on a broad range of
spatial scales. This paper first discusses consequences of the small-scale
structures associated with the strong instability inherent to the line-driving
of these winds. We demonstrate the importance of a proper treatment of such
wind clumping to obtain reliable estimates of mass-loss rates, and also show
that instability simulations that are perturbed at the lower boundary indeed
display significant clumping quite close to the wind base, in general agreement
with observations. But a growing subset of massive stars has also been found to
possess strong surface magnetic fields, which may channel the star's outflow
and induce also large-scale wind structures and cyclic behavior of spectral
diagnostics. The paper concludes by showing that multi-dimensional,
magneto-hydrodynamical wind simulations, together with detailed
radiative-transfer modeling, can reproduce remarkably well the periodic Balmer
line emission observed in slowly rotating magnetic O stars like HD191612.
