  We critically examine the best lines of evidence for grain growth in
protoplanetary disks, based on modelling of observed spectral energy
distributions and images of T Tauri and Herbig Ae stars. The data are
consistent with millimeter-sized grains near disk midplanes, and micron-sized
grains near disk surfaces. We review three channels by which grains can grow,
including direct condensation from the vapor phase, grain-grain collisional
sticking, and gravitational instability. The utility of dust in identifying as
yet unseen extrasolar planets is highlighted.
