  We introduce a new and robust approach for characterizing spatially and
temporally heterogeneous behavior within a system based on the evolution of
dynamic fuctuations once averaged over different space lengths and time scales.
We apply it to investigate the dynamics in two canonical systems as the glass
transition is approached: simulated Lennard-Jones liquids and experimental
dense colloidal suspensions. We find that in both cases the onset of glassines
is marked by spatially localized dynamic fluctuations originating in regions of
correlated mobile particles. By removing the trivial system size dependence of
the fluctuations we show that such regions contain tens to hundreds of
particles for time scales corresponding to maximally non-Gaussian dynamics.
