  UV wind line variability in OB stars appears to be universal. We review the
evidence that the variability is due to large, dense, optically thick
structures rooted in or near the photosphere. Using repeated bservations and a
simple model we translate observed profile variations into optical depth
variations and, consequently, variations in measured mass loss rates. Although
global rates may be stable, measured rates vary. Consequently, profile
variations infer how mass loss rates determined from UV wind lines vary. These
variations quantify the intrinsic error inherent in any mass loss rate derived
from a single observation. These derived rates can differ by factors of 3 or
more. Our results also imply that rates from non-simultaneous observations
(such as UV and ground based data) need not agree. Finally, we use our results
to examine the nature of the structures responsible for the variability.
