  The use of a non-one-dimensional distributed feedback, arising through Bragg
diffraction in spatially periodic systems (natural and artificial
(electromagnetic, photonic) crystals), forms the foundation for the development
of volume free electron lasers (VFELs). The present review addresses the basic
principles of VFEL theory and describes the promising potential of VFELs as the
basis for the development of high-power microwave and optical sources.
