Active libraries are libraries that play an active part in the
compilation, and especially the optimization, of their client code.
This talk motivates active libraries with a couple of examples we have
built, for linear algebra and for visual effects for the movie
industry.  The main objective is to press the argument for active
libraries as the delivery medium for performance optimisation
technologies, and to map out some of the issues.  As well as concrete
examples, I will explore some of the issues in developing tools and
interoperable infrastructure.