  We review recent developments regarding radiation driven mass loss from
OB-stars. We first summarize the fundamental theoretical predictions, and then
compare these to observational results (including the VLT-FLAMES survey of
massive stars). Especially we focus on the mass loss-metallicity dependence and
on the so-called bi-stability jump.
  Subsequently we concentrate on two urgent problems, weak winds and wind
clumping, that have been identified from various diagnostics and that challenge
our present understanding of radiation driven winds. We discuss the problems of
``measuring'' mass-loss rates from weak winds and the potential of the near
infrared, Br-alpha line as a tool to enable a more precise quantification, and
comment on physical explanations for mass-loss rates that are much lower than
predicted by the standard model.
  Wind clumping, conventionally interpreted as the consequence of a strong
instability inherent to radiative line-driving, has severe implications for the
interpretation of observational diagnostics, since derived mass-loss rates are
usually overestimated when clumping is present but ignored in the analyses.
Simplified techniques to account for clumping indicate overestimates by factors
of 2 to 10, or even more. If actually true, these results would have a dramatic
impact on the evolution of, and the feedback from, massive stars. We discuss
ongoing attempts (including own work) to interprete the corresponding
observations in terms of more sophisticated models. By allowing for porosity in
density and velocity space, and for a non-void inter-clump medium, such models
might require only moderate reductions of mass-loss rates.
