The possible applications of high-power millimeter (mm) and submillimeter waves from free electron masers (FEMs) and gyro-devices span a wide range of technologies. The plasma physics community has already taken advantage of recent advances in applying high-power micro- and mm-waves generated by longpulse or continuous wave (CW) gyrotron oscillators 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 (8 GHz), electron cyclotron resonance heating (ECRH) (28-170 GHz), electron cyclotron current drive (ECCD), collective Thomson scattering and heat-wave propagation experiments. Frequency tunable FEMs could widen these fields of applications. Another important application of CW gyrotrons is industrial materials processing, e.g. sintering of high performance functional and structural nanostructured ceramics. Submillimeter wave gyrotrons are employed in high-frequency broadband electron paramagnetic resonance (EPR) spectroscopy. Future applications which await the development ... mehrof novel high-power FEM amplifiers and gyro-amplifiers include 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). The present paper reviews the state-of-the-art and future prospects of these recent applications of gyro-devices and FEMs and compares their specific advantages.