– 2 – ABSTRACT We have obtained millimeter wavelength photometry, high-resolution opti- cal spectroscopy and adaptive optics near-infrared imaging for a sample of 26 Spitzer -selected transition circumstellar disks. All of our targets are located in the Ophiuchus molecular cloud (d ∼ 125 pc) and have Spectral Energy Distri- butions (SEDs) suggesting the presence of inner opacity holes. We use these ground-based data to estimate the disk mass, multiplicity, and accretion rate for each object in our sample in order to investigate the mechanisms potentially re- sponsible for their inner holes. We find that transition disks are a heterogeneous group of objects, with disk masses ranging from < 0.6 to 40 M$_{JUP}$ and accretion rates ranging from < 10 − 11 to 10 − 7 M$_{⊙}$yr − $^{1}$, but most tend to have much lower masses and accretion rates than “full disks” (i.e., disks without opacity holes). Eight of our targets have stellar companions: 6 of them are binaries and the other 2 are triple systems. In four cases, the stellar companions are close enough to suspect they are responsible for the inferred inner holes. We find that 9 of our 26 targets have low disk mass ( < 2.5 M$_{JUP}$ ) and negligible accretion ( < 10 − 11 M$_{⊙}$yr − $^{1}$), and are thus consistent with photoevaporating (or photoevaporated) disks. Four of these 9 non-accreting objects have fractional disk luminosities < 10 − 3 and could already be in a debris disk stage. Seventeen of our transition disks are accreting. Thirteen of these accreting objects are consistent with grain growth. The remaining 4 accreting objects have SEDs suggesting the presence of sharp inner holes, and thus are excellent candidates for harboring giant planets. Subject headings: circumstellar matter — binaries: general — planetary systems: protoplanetary disks — stars: pre-main sequence