C. Espaillat (Harvard-Smithsonian Center for Astrophysics, Cambridge, United States),
S. Andrews (Harvard-Smithsonian Center for Astrophysics, Cambridge, United States),
N. Calvet (University of Michigan, United States),
P. D'Alessio (Instituto de Astronomiia, UNAM, Mexico),
J. Hashimoto (National Astronomical Observatory of Japan, Japan),
A. Kraus (Institute for Astronomy, Hawaii, United States),
S. Kraus (Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, United States),
J. Muzerolle (Space Telescope Science Institute, United States),
J. Najita (National Optical Astronomy Observatory, United States),
Z. Zhu (Princeton University, United States)

Disks around T Tauri stars (TTS) are thought to be the sites of planet formation. However, many questions exist concerning how the gas and dust in the disk evolve into a planetary system. Observations of TTS may provide insights and there are some objects in particular that have gained increasing attention in this regard. These objects contain inner holes in their disks and have been dubbed transitional disks (TDs). Some have speculated that planets are responsible for carving out the holes in these TDs and that they are in transition between protoplanetary disks and post-planet building disks (i.e., debris disks). Spitzer produced detailed spectral energy distributions (SEDs) that allowed us to infer the radial structure of TDs in some detail, and also pointed to the diversity of TD systems. The growing sample of TDs opened up the possibility of demographic studies, which are providing unique insights. More recently, Spitzer results have stimulated work with other facilities. There now exist sub-millimeter and near-infrared (NIR) images that confirm large cavities in TDs. In addition, potential protoplanets have been detected in some of these disks. TDs are the strongest link to planet formation around TTS to date and are a key area to study if further progress is to be made on understanding the initial stages of planet formation. Here we will look at key observational properties constraining the dust and gas properties of TDs and compare them to the main disk clearing mechanisms proposed to date (i.e., photoevaporation, grain growth, and planets).

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