The AlGaN/GaN HEMT has become a relatively mature power electronic device with various versions being offered commercially, and it has already enabled significant improvements in the efficiency and power density of power conversion systems based upon it. However, it has primarily been limited to breakdown voltages less than 1.2 kV. The III-Nitride semiconductors, however, offer significant promise for medium-voltage (defined here as 1.2-20 kV) devices beyond the traditional GaN-channel HEMT. This talk will describe various efforts at Sandia National Laboratories to develop such devices, including both vertical GaN devices as well as devices based upon ultra-wide-bandgap AlGaN alloys. In addition to device results, the talk will discuss the material properties necessary to advance device performance, processing challenges, reliability, and key application areas including electric vehicles and the electric grid.