It is shown that for plasma-confinement times inferior to the Bohm diffusion time, fusion reactors are conceivable only when densities above 1019 ions/cm3 are considered. Two classes of such reactors are examined: the non-selfdestructive and the selfdestructive devices. A review is given of experiments and of theories concerning both classes (the plasma focus, the axial flow-pinch, the inertial confinement, the laser and electron-beam produced plasmas and the plasmas produced by projectile-impact). Non-fusion applications are also discussed. Reactor and trigger criteria are derived in the appendices.