  We study concentrated binary colloidal suspensions, a model system which has
a glass transition as the volume fraction $\phi$ of particles is increased. We
use confocal microscopy to directly observe particle motion within dense
samples with $\phi$ ranging from 0.4 to 0.7. Our binary mixtures have a
particle diameter ratio $d_S/d_L=1/1.3$ and particle number ratio
$N_S/N_L=1.56$, which are chosen to inhibit crystallization and enable
long-time observations. Near the glass transition we find that particle
dynamics are heterogeneous in both space and time. The most mobile particles
occur in spatially localized groups. The length scales characterizing these
mobile regions grow slightly as the glass transition is approached, with the
largest length scales seen being $\sim 4$ small particle diameters. We also
study temporal fluctuations using the dynamic susceptibility $\chi_4$, and find
that the fluctuations grow as the glass transition is approached. Analysis of
both spatial and temporal dynamical heterogeneity show that the smaller species
play an important role in facilitating particle rearrangements. The glass
transition in our sample occurs at $\phi_g \approx 0.58$, with characteristic
signs of aging observed for all samples with $\phi>\phi_g$.
