Stars are all rotating but some more than others... Rotation is fast when you cannot ignore it, whatever the problem you are considering. Practically, I consider that if the
centrifugal flattening of a star exceeds 5%, it is rapidly rotating. The Sun's flattening is 0.001%, thus very very small: it is a slowly rotating star.
In this talk I will therefore focus on stars that spin about a hundred times faster than the Sun. I'll present the many questions, and their first answers, emerging as we seek to
understand how rotation shapes stellar evolution and stellar seismology.