We have been using planetary nebulae (PNe) to probe the kinematic structure of face-on spiral disks by identifying large (∼100) samples of these objects via narrow-band imaging, and then measuring their radial velocities with follow-up, high-precision (∼5 km s-1) spectroscopy. Our results for IC 342, M74, M83, M94, and M101 are quite interesting. With one exception (M101) the z-velocity dispersion (σz) of galactic disks declines exponentially with the light out to ∼3 disk scale lengths. This is as expected for a constant M/L, constant scale height disk. However, in two galaxies with significant data past this radius, the values of σz asymptote out at ∼20 km s-1. Moreover, our analysis finds kinematic evidence for significant flaring in the outer regions as well, especially in M94. These observations are in excellent agreement with predictions derived from models of disk accretion of subhalos, and demonstrate how kinematic surveys in the outer disks of spirals can be used to test hierarchical models of galaxy formation. We also find that the disks of late-type galaxies are far from maximal, that the disks of early-type spirals have higher M/L ratios than the disks of later-type objects, and that the unseen inner halos of spiral galaxies are better fit by pseudo-isothermal laws than by NFW models.