  Transitional disks are objects whose inner disk regions have undergone
substantial clearing. The Spitzer Space Telescope produced detailed spectral
energy distributions (SEDs) of transitional disks that allowed us to infer
their radial dust disk structure in some detail, revealing the diversity of
this class of disks. The growing sample of transitional disks also opened up
the possibility of demographic studies, which provided unique insights. There
now exist (sub)millimeter and infrared images that confirm the presence of
large clearings of dust in transitional disks. In addition, protoplanet
candidates have been detected within some of these clearings. Transitional
disks are thought to be a strong link to planet formation around young stars
and are a key area to study if further progress is to be made on understanding
the initial stages of planet formation. Here we provide a review and synthesis
of transitional disk observations to date with the aim of providing timely
direction to the field, which is about to undergo its next burst of growth as
ALMA reaches its full potential. We discuss what we have learned about
transitional disks from SEDs, color-color diagrams, and imaging in the (sub)mm
and infrared. We then distill the observations into constraints for the main
disk clearing mechanisms proposed to date (i.e., photoevaporation, grain
growth, and companions) and explore how the expected observational signatures
from these mechanisms, particularly planet-induced disk clearing, compare to
actual observations. Lastly, we discuss future avenues of inquiry to be pursued
with ALMA, JWST, and next generation of ground-based telescopes.
