Future development and applications of accelerators and electron devices in the fields of free electron lasers (FELs), high-frequency vacuum electronics, high-power microwave devices, and electron beam lithography are partially restricted due to the electron source brightness. A high brightness electron gun with small emittance and high average beam current is one of the key goals in current technological developments. RF electron guns have the ability to reduce emittance growth due to the space-charge effects. Among RF guns, thermionic guns are quite attractive because of their simple configuration with no high voltage stage or complicated laser systems. In addition, the high acceleration reached in the gun cavity (up to 100 MV/m) can easily compress the bunch lengths to have sufficient peak current for FEL driving. On the other hand, Back Bombardment (BB) effect limits the wide usage of Thermionic RF guns worldwide. In this book, BB phenomena, associated effects and method of mitigations are presented. Numerical Model for simulating the effect of back bombardment in thermionic RF gun is used to discus the phenomeno