Abstract

Rapid progress is being made in developing observational constraints on the structure and evolution of protoplanetary disks, primarily due advances in spectral sensitivity due to the Spitzer Space Telescope and improvements in spatial resolution from mm-wave interferometry. Unfortunately there still are considerable uncertainties in the masses and mass distributions of disks. I will review the observational limits we now have on disk masses, the evidence for dust growth and settling in these disks, and the increasingly common indications of inner disk clearing at early evolutionary times (~ 1 Myr). I will then argue that the time-dependence of disk accretion in the protostellar phase strongly suggests that something like a dead zone or high-surface-density, low-viscosity, region exists in inner disks, at least initially, providing the conditions for more rapid formation of relatively massive bodies.