AbstractPhotometric and/or spectroscopic variability on time scales of approximately 0.2 to 2 days has been observed in over 40 Be stars, and is suspected in many more. This paper reviews the observational aspects of this phenomenon: both surveys and studies of individual objects. This phenomenon is not easy to study and interpret: (i) the time scale is inconvenient (ii) there have been very few simultaneous photometric and spectroscopic studies of individual objects (iii) the photometric

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... ility is small, often irregular and superimposed on longer-term variability and (iv) the spectroscopic variability is usually observed as absorption line profile variability, which requires special instrumentation. For these and other reasons, there is not yet a universal agreement about the nature of this phenomenon. Nevertheless, it deserves further intensive study, not only because it is common, but also because in one or two stars, there is evidence that the rapid variability may be related (causally perhaps?) to the longer-term variability in these stars - variability whose ultimate cause is still not known.