  Dynamic heterogeneity is now recognised as a central aspect of structural
relaxation in disordered materials with slow dynamics, and was the focus of
intense research in the last decade. Here we describe how initial, indirect
observations of dynamic heterogeneity have recently evolved into well-defined,
quantitative, statistical characterisations, in particular through the use of
high-order correlation and response functions. We highlight both recent
progress and open questions about the characterisation of dynamic heterogeneity
in glassy materials. We also discuss the limits of available tools and describe
a few candidates for future research in order to gain deeper understanding of
the origin and nature of glassiness in disordered systems.
