In recent years many observations of short time scale variability in a variety of spectral classes have been published. Some of these observations relate to phenomena which are reasonably well understood, as for example the Beta Canis Majoris stars, which are short period pulsators. In other cases, such as the Of and WR stars, the mechanism responsible for the variation is not fully understood. Models proposed to explain this variability include:- the binary hypothesis which ascribes the variations to fluctuations in gas streams in and around the members of a close binary system, and:- the pulsation hypothesis, which ascribes the variation in line strength to the presence of pulsational instability of massive carbon burning cores of evolved objects. The choice of the correct model is made difficult by the lack of an extended set of homogeneous observations. We must fulfil two conditions before making this choice. It is necessary to observe individual objects over extended periods of time at high dispersion to establish whether or not all WR's are binary systems. It is also required firmly to establish the temporal nature of the variations, since short term periodicities would tend to favor the core pulsation model.