The x‐ray source in high mass x‐ray binaries (HMXB’s) provides a unique ‘‘active’’ probe of the winds of massive stars. As the binary system orbits around the center of mass, our line of sight changes continuously, allowing us to map the distribution of x‐ray absorbing gas in the system. At the same time, the strong x‐ray flux alters the local dynamics of the wind, providing us with an opportunity to study the physics of radiatively‐driven winds. Some HMXB systems even provide a variable x‐ray source, from which we can measure the effects of a local x‐ray flux. New hydrodynamic models, coupled with recent observations, allow us to take advantage of this natural x‐ray machine, providing new insight into the dynamics of radiatively driven winds. The various physical processes that affect the wind dynamics will be reviewed, as well as the observational consequences of these processes as gleaned from multidimensional hydrodynamic models. ©1994 American Institute of Physics.