disks are rare, it is likely that all circumstellar disks go through a short transition disk stage (as defined by their SEDs) at some point of their evolution. This is so because observations show that an IR excess at a given wavelength is always accompanied by an excess at longer wavelengths (the converse is not true as attested by the very existence of transition disks). 

This implies that, unless some disks manage to lose the near-, mid- and far-IR excess at exactly the same time, the near-IR excess always dissipates before the mid-IR and far-IR excess do. Since no known process is expected to remove the circumtellar dust at all radii simultaneously, it is reasonable to conclude that transition disks represent a common (if not unavoidable) phase in the evolution of a circumstellar disk. 

At least four different mechanisms have been proposed to explain the opacity holes of transition disks: giant planet formation, grain growth, photoevaporation, and tidal truncation in close binaries. However, since all these processes can in principle result in similar IR SEDs, additional observational constraints are necessary to distinguish among them. As discussed by Najita et al. (2007), Cieza (2008), and Alexander (2008), disk masses, accretion rates, and multiplicity information are particularly useful to distinguish between the different mechanisms that are likely to produce the inner holes in transition disks. A vivid example of the need for these kind of data is the famous disk around CoKu Tau/4. Since its sharp inner hole was discovered by Spitzer (Forrest et al. 2004), its origin has been a matter of great debate. The hole was initially modeled to be carved by a giant planet (Quillen et al. 2004), but Najita et al. (2007) argued that the low mass and low accretion rate of the CoKu Tau/4 disk are more consistent with photoevaporation than with a planet formation scenario. More recently, Coku Tau/4 has has been shown to be a close binary star system with a 8 AU projected separation, rendering its disk a circumbinary one (Ireland & Kraus 2008). 

Since the number of well characterized transition disks is still in the tens, most studies 

