– 4 – Recent submillimeter studies provide dramatic support for the presence of inner opacity holes inferred from the SED modeling of transition disks. High resolution submillimeter continuum images of objects such as LkH α 330 (Brown et al. 2008) and GM Tau (Hughes et al. 2009) show sharp inner holes tens of AU in radius and lend confidence to the standard interpretation of transition disk SEDs. One of the most intriguing results from transition disk studies has been the great diversity of SED morphologies revealed by the Spitzer Space Telescope. In an attempt to describe distinct classes of transition disks, several names have recently emerged in the literature (Evans et al. 2009). These new names include: anemic disks, flat disks, or homologously depleted disks to describe objects whose observed SEDs are significantly below the median of the CTTS population at all IR wavelengths (Lada et al. 2006, Currie et al. 2009). These new names also include cold disks, objects with little or no near-IR excesses whose SEDs raise very steeply in the mid-IR (Brown et al. 2007), and pre-transitional disks, disks with evidence for an optically-thin gap separating optically-thick inner and outer disk components (Espaillat et al. 2008). Studying the diverse population of transition disks is key for understanding circumstellar disk evolution as much of the diversity of their SED morphologies is likely to arise from different physical processes dominating the evolution of different disks. Disk evolution processes include: viscous accretion (Hartmann et al. 1998), photoevaporation (Alexander et al., 2006), the magnetorotational instability (Chiang & Murray-Clay, 2007), grain growth and dust settling (Dominik & Dullemond, 2008), planet formation (Lissauer, 2003; Boss et al. 2000), and dynamical interactions between the disk and stellar or substellar companions (Artymowicz & Lubow, 1994). Understanding the relative importance of these physical processes in disk evolution and their connection to the different classes of transition disks is currently one of the main challenges of the disk evolution field. Also, even though transition