  Over times shorter than that required for relaxation of enthalpy, a liquid
can exhibit striking heterogeneities. The picture of these heterogeneities is
complex with transient patches of rigidity, irregular yet persistent,
intersected by tendrils of mobile particles, flickering intermittently into new
spatial patterns of motion and arrest. The study of these dynamic
heterogeneities has, over the last 20 years, allowed us to characterize
cooperative dynamics, to identify new strategies in controlling kinetics in
glass-forming liquids and to begin to systematically explore the relationship
between dynamics and structure that underpins the behaviour of amorphous
materials. Computer simulations of the dynamics in atomic and molecular liquids
have played a dominant role in all of this progress. While some may be uneasy
about this reliance on modelling, it is unavoidable, given the amount of
microscopic detail needed to characterize the dynamic heterogeneities. The
complexities revealed by these simulations have called for new conceptual
tools. In this essay, I have tried to provide the reader with a clear and
complete account of how these tools have been developed in terms of the
literature on kinetic length scales in molecular dynamics simulations. Through
the `prism' of these length scales, this essay addresses the question what have
we learnt about dynamic heterogeneities from computer simulations?
