We outline the fundamental processes of coherent radiation emission 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 N, a pre-bunched electron beam emits spontaneously coherent radiation proportional to N2 through the process of (spontaneous) superradiance (SP-SR) (in the sense of Dicke's ). The 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. These coherent radiation emission processes are presented in term of a radiation mode expansion model, applied to general free electron radiation schemes: Optical-Klystron, HGHG, EEHG, and coherent THz sources based on synchrotron radiation, undulator radiation or Smith-Purcell radiation. The general model of coherent spontaneous emission is also extended to the nonlinear regime - Tapering Enhanced Stimulated Superradiance (TESSA) , and related to the tapered wiggler section of seed-injected FELs. In X-Ray FELs, these processes are convoluted with other effects, but they are guidelines for strategies of wiggler tapering efficiency enhancement.