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 gyrotrons in the areas of RF plasma production, heating, non-inductive current drive, plasma stabilization and active plasma diagnostics for magnetic confinement thermonuclear fusion research, such as lower hybrid current drive (LHCD) (8 G Hz), electron cyclotron resonance heating (ECRH) (28-170 G Hz), elec­ tron cyclotron current drive (ECCD), collective Thomson scattering (CTS) and heat-wave propagation experiments. Other important applications of gy­ rotrons are electron cyclotron resonance (ECR) discharges for the generation of multi-charged ions and soft X-rays, as well as industrial materials process­ ing and plasma chemistry. Submillimeter wave gyrotrons are employed in high frequency, broadband electron paramagnetic resonance (EPR) spectroscopy. Additional future applications await the development of novel high power gyro-amplifiers and devices for high resolution radar ranging and imaging in atmospheric and planetary science as well as deep space and specialized satellite communications and RF drivers for next generation high gradient linear accelerators (supercolliders).