Observations and modeling of gaseous protoplanetary disks

Theoretical models and dust-continuum observation of protoplanetary disks are briefly reviewed. We focus on the thermal structure of the disk, the effect of grain growth and sedimentation on disk structure, and spatial variation of chemical ( molecular) abundances. Observations of the gaseous component are reviewed, ranging from UV, optical and infrared emissions to millimeter lines. The former arises mostly from the hot or warm gas in the inner radius (r <= a few tens of AU), whereas the latter arises from the outer radius. Gas observation can probe the disk structure independently of dust observation. Combining the line and continuum observations is thus very important. Gas observation can also reveal the kinematics, ionization and chemistry in protoplanetary disks.