Gyrotron oscillators (gyrotrons) are capable of providing hundreds of kilo- watts of power at microwave and millimetric wavelengths. From their con- ception in the late fifties until their successful development for various ap- plications, gyrotrons have come a long way technologically and made an ir- reversible impact on both users and developers. The possible applications of high power millimeter and sub-millimeter waves from gyrotrons and their variants (gyro-devices) span a wide range of technologies. The plasma physics community has already taken advantage of the recent advances of...