The basic principles of free electron generators of coherent radiation and
the practical considerations related to the development of far infrared and
sub-millimeter wave sources are discussed in this paper. The attention is
focused on compact devices, which reduce the size and cost issue. A review
of the main results obtained at the Frascati FEL Facility is presented toge
ther with the planned experiments. (C) 1999 Elsevier Science B.V. All right
s reserved.