  A transiting planet eclipses part of the rotating stellar surface, thereby
producing an anomalous Doppler shift of the stellar spectrum. Here I review how
this "Rossiter-McLaughlin Effect" can be used to characterize exoplanetary
systems. In particular, one can measure the angle on the sky between the
orbital axis and the stellar rotation axis. This may help to discriminate among
migration theories. Measurements have been made for 4 exoplanets, and in all
cases the spin and orbital axes are fairly well-aligned. In the future, the
Rossiter-McLaughlin effect may also be important as an alternative means of
probing exoplanetary atmospheres, and for confirming the transits of objects
identified by the satellite missions Corot and Kepler.
