The last decade, extensive spectroscopic monitoring campaigns, often combining ground-based (optical) and space-born (ultraviolet) observations, of OB-type stars have shown that their stellar winds are strongly variable. The most prominent features of wind variability are the so-called discrete absorption components (DACs), which migrate through the UV P Cygni profiles on a timescale of hours to days. One of the major breakthroughs in this field of research has been the recognition that wind variability is cyclical in nature. This suggests that stellar rotation is an important piece in the unsolved puzzle of the wind-variability mechanism. The current idea is that Corotating Interacting Regions (CIRs) are responsible for the observed wind variations. In this model, the large-scale structure in the stellar wind is caused by the interaction of fast and slow streams that originate at neighbouring locations on the stellar surface. Due to the rotation of the star the streams are curved, causing fast wind material to collide with slow material in front. The interaction region has a spiral shape and corotates with the star. In order to work, the CIR model needs a certain structure imposed at the stellar surface to produce flows with different kinematic properties. Two candidate physical mechanisms are: (i) non-radial pulsations (NRP), or (ii) surface magnetic fields. The search for the photospheric connection is the subject of current investigations.