Recent results will be presented on high frequency (100-300 GHz) megawatt gyrotron experiments being conducted at MIT. Two gyrotron oscillators are being investigated: one operating at 148 GHz in the TE16,2,1 mode, and the other at 280 GHz in the TE42,7,1 mode. These gyrotrons operate with 3μsec pulses, but can be scaled to cw operation. The highest power at 148 GHz has been achieved with a two-section cavity. Powers up to 1.2 MW with an efficiency of 32% were measured at 48 A. The 280 GHz experiment will begin shortly, and will use a single cavity with a diameter of 23 wavelengths. This will be a severe test of the gyrotron's ability to excite a single mode and suppress competing modes. Measurements of the electron beam spatial and velocity characteristics for the 140 GHz gun will also be presented.