  We outline the fundamental coherent radiation emission processes from a
bunched charged particles beam. In contrast to spontaneous emission of
radiation from a random electron beam that is proportional to the number of
particles, a pre-bunched electron beam can emit spontaneously coherent
radiation proportional to the number of particles - squared, through the
process of (spontaneous) superradiance (SP-SR) (in the sense of Dicke's). The
coherent SP-SR emission of a bunched electron beam can be even further enhanced
by a process of stimulated-superradiance (ST-SR) in the presence of a seed
injected radiation field. In this review, these coherent radiation emission
processes for both single bunch and periodically bunched beams are considered
in a model of radiation mode expansion.
  The general model of coherent spontaneous emission is extended to the
nonlinear regime, particularly for undulator (wiggler) interaction:
Tapering-Enhanced Stimulated Superradiant Amplification (TESSA).
  Processes of SP-SR and TESSA take place in tapered wiggler seed-injected
FELs. In such FELs, operating in the X-Ray regime, these processes are
convoluted with other effects. However these fundamental emission concepts are
useful guidelines for the strategy of wiggler tapering efficiency and power
enhancement.
  Based on this model, we review previous theories and experiments on coherent
radiation sources based on SP-SR (coherent undulator radiation, synchrotron
radiation, Smith-Purcell radiation etc.), in the THz regime and on-going works
on tapered wiggler efficiency-enhancement concepts in all optical frequency
regimes up to X-Rays.
